Add ability to control playback using command-line flags

[dweymouth]

Adds the following command-line options

> ./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>)
  -version
        print app version and exit
  -volume value
        sets the playback volume (0-100)