plugdj issue 9 - Add optional arguments to /leaveafter

Enables leaving after [1-9] tracks.

Without arguments, leaves the DJ booth or waiting list after you play
your next song.  If you are currently playing, it will leave after this
song.  With an int argument, leave after you have played that number of
tracks.  Also, supports the arg -1 or cancel to cancel the leaveafter cue.