Add App.InvalidFlagAccessHandler (#1446)

* Add App.UnknownFlagHandler

* Rename App.UnknownFlagHandler to App.InvalidFlagAccessHandler

* Traverse parent contexts

app.go

@@ -78,6 +78,8 @@ type App struct {
 	CommandNotFound CommandNotFoundFunc
 	// Execute this function if a usage error occurs
 	OnUsageError OnUsageErrorFunc
+	// Execute this function when an invalid flag is accessed from the context
+	InvalidFlagAccessHandler InvalidFlagAccessFunc
 	// Compilation date
 	Compiled time.Time
 	// List of all authors who contributed