implement *playback commands other than PLAY, in particular LIST

[slingamn]

Not supporting LIST breaks Palaver. This should be tested with Textual and Palaver.

[slingamn]

From discussion: we'll also add *self to mean "play all direct messages, but no channel messages".

[slingamn]

Palaver sends the command as one parameter: ZNC *playback :PLAY ...