Register a handler that's called after a channel is joined.
        The handler is called with the `Channel` as argument, must be a coroutine
        and is run non-blocking.
        :param channel: channel to look out for or `None` for all channels

def on_join(self, channel: str=None) -> Callable[[Callable], Callable]:
        """
        Register a handler that's called after a channel is joined.
        The handler is called with the `Channel` as argument, must be a coroutine
        and is run non-blocking.
        :param channel: channel to look out for or `None` for all channels
        """
        def decorator(fn: Callable[[self.IrcMessage], None]):
            jh = self.JoinHandler(channel, fn)
            self._on_join_handlers.append(jh)
            self._log.debug("Added join handler {}".format(jh))
            return fn

        return decorator