Allow global logging when developing

I'll be honest, for all I know logging should be global in general: it is
a pain to pass a logger to any struct that needs it. But smart people on the
internet tell me otherwise, and I do like the idea of not having any global
variables lying around.

Nonetheless, I often need to log things when locally debugging and that's a
different kind of logging than the kind you would include in the actual
released binary. For example if I want to log something from gocui, I would
rather not have gocui depend on lazygit's logging setup.

pkg/config/app_config.go

@@ -301,5 +301,9 @@ func getDefaultAppState() *AppState {
 }
 
 func LogPath() (string, error) {
+	if os.Getenv("LAZYGIT_LOG_PATH") != "" {
+		return os.Getenv("LAZYGIT_LOG_PATH"), nil
+	}
+
 	return configFilePath("development.log")
 }