support MPD client-to-client channels

[k80w]

it would be great if this package supported client-to-client channels. i haven't looked at this package in too much detail yet, so i don't know how difficult this would be to implement.

[k80w]

With the way this project is structured, the most sensible option to me would be to create a Subscriber type that functions similarly to the Watcher type, but rather than idleing a list of subsystems, subscribes to a list of channels, idles the message subsystem, and returns results of readmessages on any message event.