plugdj issue 9 - Add optional arguments to /leaveafter, enables leaving after n 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.
Refactor leaveafter logic.  This better suits querying local storage for leaveafter count.
tweaking some of samb0t's changes
fixing some syntax errors I introduced and a couple other small bugs with the leaveafter code
updating help text for /leaveafter
bumping version number
Merge branch 'master' of https://github.com/thetoast/plugjs