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/eventtypes/event/event.go

@@ -77,7 +77,7 @@ func (b *Base) GetReasons() []string {
 	return b.Reasons
 }
 
-// GetBase returns an event base
+// GetBase returns the underlying base
 func (b *Base) GetBase() *Base {
 	return b
 }