More music controls

[lucawen]

Add controls:

• Skip all songs from queue (skip_all or ska)
• When start 'play' a song or playlist, if not already in channel, connect before play.
• New Play command for play a song/playlist and override current queue (current play command add to queue. playnow or plw)

Fix:

• The queue loses itself of automatic play sometimes, so detect when it is not playing anything;
• Detect when the voice channel is empty and get out from channel;