Add 'start' event that is triggered once when fzf finder starts

Close #1622

src/options.go

@@ -543,6 +543,8 @@ func parseKeyChords(str string, message string) map[tui.Event]string {
 			add(tui.Change)
 		case "backward-eof":
 			add(tui.BackwardEOF)
+		case "start":
+			add(tui.Start)
 		case "alt-enter", "alt-return":
 			chords[tui.CtrlAltKey('m')] = key
 		case "alt-space":

src/terminal.go

@@ -2362,13 +2362,21 @@ func (t *Terminal) Loop() {
 	}()
 
 	looping := true
+	_, startEvent := t.keymap[tui.Start.AsEvent()]
+
 	for looping {
 		var newCommand *string
 		changed := false
 		beof := false
 		queryChanged := false
 
-		event := t.tui.GetChar()
+		var event tui.Event
+		if startEvent {
+			event = tui.Start.AsEvent()
+			startEvent = false
+		} else {
+			event = t.tui.GetChar()
+		}
 
 		t.mutex.Lock()
 		previousInput := t.input

src/tui/tui.go

@@ -90,6 +90,7 @@ const (
 
 	Change
 	BackwardEOF
+	Start
 
 	AltBS