[Decorator] Add handler method.

        :param event: Specify a kind of Event which you want to handle
        :type event: T <= :py:class:`linebot.models.events.Event` class
        :param message: (optional) If event is MessageEvent,
            specify kind of Messages which you want to handle
        :type: message: T <= :py:class:`linebot.models.messages.Message` class
        :rtype: func
        :return: decorator

def add(self, event, message=None):
        """[Decorator] Add handler method.

        :param event: Specify a kind of Event which you want to handle
        :type event: T <= :py:class:`linebot.models.events.Event` class
        :param message: (optional) If event is MessageEvent,
            specify kind of Messages which you want to handle
        :type: message: T <= :py:class:`linebot.models.messages.Message` class
        :rtype: func
        :return: decorator
        """
        def decorator(func):
            if isinstance(message, (list, tuple)):
                for it in message:
                    self.__add_handler(func, event, message=it)
            else:
                self.__add_handler(func, event, message=message)

            return func

        return decorator