Config: refactor config file loaders (#577)

* Config: fix don't create empty dir when resolving path

* Config: refactor config file loaders

* add a layer of abstraction so that config can be loaded from non-files
* use io.Reader / io.Writer abstraction to separate data operations from
file operations
* remove dryrun option from SaveConfig - now it always saves

* rename read and save methods to mention file operations

* log error when encryption prompt fails

* as the user didn't make a choice, we'd prompt again next time the file
is loaded
* add file.Writer tests
* skip permissions test for windows

* defer creating the writer on save to the last moment

* this avoids truncating file when there is error with password prompt
* add a test

* tests with StdIn cannot run in parallel

cmd/exchange_template/exchange_template.go

@@ -227,7 +227,7 @@ func makeExchange(exch *exchange) error {
 	}
 
 	configTestFile.Exchanges = append(configTestFile.Exchanges, newExchConfig)
-	err = configTestFile.SaveConfig(exchangeConfigPath, false)
+	err = configTestFile.SaveConfigToFile(exchangeConfigPath)
 	if err != nil {
 		return err
 	}

cmd/exchange_template/exchange_template_test.go

@@ -68,7 +68,7 @@ func TestNewExchange(t *testing.T) {
 		t.Fatalf("unable to remove exchange config for %s, manual removal required\n",
 			testExchangeName)
 	}
-	if err := cfg.SaveConfig(exchangeConfigPath, false); err != nil {
+	if err := cfg.SaveConfigToFile(exchangeConfigPath); err != nil {
 		t.Fatal(err)
 	}
 }