backtester: custom strategy plugins (#989)

* Adds custom strategy

* docs and structure

* docs

* rn

* Documents plugins, adds custom strat config

* mini fixes. Fleshes strategy test

* docgen

* Updates plugins to allow for multiple strategies to be loaded

* docs

* docs regen

* fix doc accuracy

* why did I add the word custom?

backtester/plugins/strategies/example/README.md

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+# GoCryptoTrader Backtester: Example package
+
+<img src="/backtester/common/backtester.png?raw=true" width="350px" height="350px" hspace="70">
+
+
+[![Build Status](https://github.com/thrasher-corp/gocryptotrader/actions/workflows/tests.yml/badge.svg?branch=master)](https://github.com/thrasher-corp/gocryptotrader/actions/workflows/tests.yml)
+[![Software License](https://img.shields.io/badge/License-MIT-orange.svg?style=flat-square)](https://github.com/thrasher-corp/gocryptotrader/blob/master/LICENSE)
+[![GoDoc](https://godoc.org/github.com/thrasher-corp/gocryptotrader?status.svg)](https://godoc.org/github.com/thrasher-corp/gocryptotrader/backtester/plugins/strategies/example)
+[![Coverage Status](http://codecov.io/github/thrasher-corp/gocryptotrader/coverage.svg?branch=master)](http://codecov.io/github/thrasher-corp/gocryptotrader?branch=master)
+[![Go Report Card](https://goreportcard.com/badge/github.com/thrasher-corp/gocryptotrader)](https://goreportcard.com/report/github.com/thrasher-corp/gocryptotrader)
+
+
+This example package is part of the GoCryptoTrader codebase.
+
+## This is still in active development
+
+You can track ideas, planned features and what's in progress on this Trello board: [https://trello.com/b/ZAhMhpOy/gocryptotrader](https://trello.com/b/ZAhMhpOy/gocryptotrader).
+
+Join our slack to discuss all things related to GoCryptoTrader! [GoCryptoTrader Slack](https://join.slack.com/t/gocryptotrader/shared_invite/enQtNTQ5NDAxMjA2Mjc5LTc5ZDE1ZTNiOGM3ZGMyMmY1NTAxYWZhODE0MWM5N2JlZDk1NDU0YTViYzk4NTk3OTRiMDQzNGQ1YTc4YmRlMTk)
+
+## Example package overview
+
+This is a custom strategy for the GoCryptoTrader Backtester. It is a simple example of a strategy that trades a pair of assets and is used to highlight how strategies can be loaded from external sources.
+
+### Designing a strategy
+- File must contain `main` package.
+- Custom strategy plugins must adhere to the strategy.Handler interface. See the [strategy.Handler interface documentation](./backtester/eventhandlers/strategies/README.md) for more information.
+- Must contain function `func GetStrategies() []strategy.Handler` to return a slice of implemented `strategy.Handler`.
+   - If only using one custom strategy, can simply `return []strategy.Handler{&customStrategy{}}`.
+
+### Building
+See [here](./backtester/plugins/README.md) for details on how to build the plugin file.
+
+### Running
+Plugins can only be loaded via Linux, macOS and WSL. Windows itself is not supported.
+
+To run this strategy you will need to use the following flags when running the GoCryptoTrader Backtester:
+
+```bash
+./backtester -strategypluginpath="path/to/strategy/example.so"
+```
+
+To run this specific example strategy, use:
+
+```bash
+./backtester --strategypluginpath="./plugins/strategies/example/example.so"
+```
+
+Upon startup, the GoCryptoTrader Backtester will load the strategy and run it for all events.
+
+
+### Please click GoDocs chevron above to view current GoDoc information for this package
+
+## Contribution
+
+Please feel free to submit any pull requests or suggest any desired features to be added.
+
+When submitting a PR, please abide by our coding guidelines:
+
++ Code must adhere to the official Go [formatting](https://golang.org/doc/effective_go.html#formatting) guidelines (i.e. uses [gofmt](https://golang.org/cmd/gofmt/)).
++ Code must be documented adhering to the official Go [commentary](https://golang.org/doc/effective_go.html#commentary) guidelines.
++ Code must adhere to our [coding style](https://github.com/thrasher-corp/gocryptotrader/blob/master/doc/coding_style.md).
++ Pull requests need to be based on and opened against the `master` branch.
+
+## Donations
+
+<img src="https://github.com/thrasher-corp/gocryptotrader/blob/master/web/src/assets/donate.png?raw=true" hspace="70">
+
+If this framework helped you in any way, or you would like to support the developers working on it, please donate Bitcoin to:
+
+***bc1qk0jareu4jytc0cfrhr5wgshsq8282awpavfahc***

backtester/plugins/strategies/example/example-strategy.go

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+package main
+
+import (
+	"github.com/thrasher-corp/gocryptotrader/backtester/data"
+	"github.com/thrasher-corp/gocryptotrader/backtester/eventhandlers/portfolio"
+	"github.com/thrasher-corp/gocryptotrader/backtester/eventhandlers/strategies"
+	"github.com/thrasher-corp/gocryptotrader/backtester/eventhandlers/strategies/base"
+	"github.com/thrasher-corp/gocryptotrader/backtester/eventtypes/signal"
+	"github.com/thrasher-corp/gocryptotrader/backtester/funding"
+	gctorder "github.com/thrasher-corp/gocryptotrader/exchanges/order"
+)
+
+func main() {
+	// required for plugin system
+}
+
+// CustomStrategy the type used to define custom strategy functions
+type CustomStrategy struct {
+	base.Strategy
+}
+
+// GetStrategies is required to load the strategy or strategies into the GoCryptoTrader Backtester
+func GetStrategies() []strategies.Handler {
+	return []strategies.Handler{&CustomStrategy{}}
+}
+
+// Name returns the name of the strategy
+func (s *CustomStrategy) Name() string {
+	return "custom-strategy"
+}
+
+// Description describes the strategy
+func (s *CustomStrategy) Description() string {
+	return "this is a demonstration of loading strategies via custom plugins"
+}
+
+// SupportsSimultaneousProcessing this strategy only supports simultaneous signal processing
+func (s *CustomStrategy) SupportsSimultaneousProcessing() bool {
+	return true
+}
+
+// OnSignal handles a data event and returns what action the strategy believes should occur
+func (s *CustomStrategy) OnSignal(d data.Handler, _ funding.IFundingTransferer, _ portfolio.Handler) (signal.Event, error) {
+	return s.createSignal(d)
+}
+
+// OnSimultaneousSignals analyses multiple data points simultaneously, allowing flexibility
+// in allowing a strategy to only place an order for X currency if Y currency's price is Z
+func (s *CustomStrategy) OnSimultaneousSignals(d []data.Handler, f funding.IFundingTransferer, p portfolio.Handler) ([]signal.Event, error) {
+	response := make([]signal.Event, len(d))
+	for i := range d {
+		sig, err := s.createSignal(d[i])
+		if err != nil {
+			return nil, err
+		}
+		response[i] = sig
+	}
+	return response, nil
+}
+
+func (s *CustomStrategy) createSignal(d data.Handler) (*signal.Signal, error) {
+	es, err := s.GetBaseData(d)
+	if err != nil {
+		return nil, err
+	}
+
+	sig := &signal.Signal{
+		Base:       es.Base,
+		OpenPrice:  es.OpenPrice,
+		HighPrice:  es.HighPrice,
+		LowPrice:   es.LowPrice,
+		ClosePrice: es.ClosePrice,
+		Volume:     es.Volume,
+		BuyLimit:   es.BuyLimit,
+		SellLimit:  es.SellLimit,
+		Amount:     es.Amount,
+		Direction:  gctorder.Buy,
+	}
+	sig.AppendReasonf("Signalling purchase of %s", es.Base.Pair())
+	return sig, nil
+}
+
+// SetCustomSettings can override default settings
+func (s *CustomStrategy) SetCustomSettings(map[string]interface{}) error {
+	return base.ErrCustomSettingsUnsupported
+}
+
+// SetDefaults sets default values for overridable custom settings
+func (s *CustomStrategy) SetDefaults() {}