Added option for automatic playback of mpd stream.

Added option for automatic playback of mpd stream.

When activated in settings, ympd will atempt to play the mpd stream
(ogg/mp3) locally in the browser whenever mpd is playing.

This allows to run a headless instance on a PC with sound and start/stop
playback also from other instances or mpd clients.
e.g. run ympd with autoplay on your private PC with decent loudspeakers,
    but control it with another instance on your office laptop without
    the need to switch PCs.