Continuous playback

[quigybo]

The music should never stop!

Once one playlist has ended, the next mix in the set should
automatically start. Pull request #12 adds bindings for the next_mix
method, and Issue #13 needs to be resolved first.

Then maybe a boolean config key for continuous play that can be toggled
via a command in the client? Thoughts?

[dbrgn]

I think the configuration should go to a config file. Such a config file was added in b119999.

[dbrgn]

Fixed by 7289a47 in #12.