Add ability to control playback using command-line flags

[dweymouth]

Adds the following command-line options (along with a follow-up commit)

> ./supersonic --help
Usage of ./supersonic:
  -help
    	print command line options and exit
  -next
    	seek to next track
  -pause
    	pause playback
  -play
    	unpause or begin playback
  -play-pause
    	toggle play/pause state
  -previous
    	seek to previous track or beginning of current
  -seek-to value
    	seeks to the given position in seconds in the current file (0.0 - <trackDur>)
  -seek-by value
        seeks forward or backward by the given time in seconds (+/-)
  -version
    	print app version and exit
  -volume value
    	sets the playback volume (0-100)