Flagger should provide a way to check if something is a valid flag
Currently we force all flaggers to have (at least) 3 flags: flagger.UNFLAGGED, flagger.BAD and flagger.GOOD. Apart from that, we don't have good way, to 'ask' the flagger if something is actually a valid flag. Maybe we should change that with a method like flagger.isFlag (pretty close to flagger.isFlagged, I guess) or flagger.isValid or flagger.isValidFlag or whatever.