Add links in prompt_yesno. Add prompt_choice.

Add links in prompt_yesno.

Put ```self.caller.cmdset.delete('menucmdset')``` before ```self.yesfunc(self) if self.nofunc else None```, so that users can use another prompt_yesno in yesfunc.

Add prompt_choice, which is similar to prompt_yesno. Users can create choice with it.
This documentation does not seem to match the function call signature.