Add a responder of last resort.

        Like `.autoresponds`, but instead of adding a responder to the top of
        the stack, add it to the bottom. This responder will be called if no
        others match.

def append_responder(self, matcher, *args, **kwargs):
        """Add a responder of last resort.

        Like `.autoresponds`, but instead of adding a responder to the top of
        the stack, add it to the bottom. This responder will be called if no
        others match.
        """
        return self._insert_responder("bottom", matcher, *args, **kwargs)