Implementing Issue #10

Yo,

This may be a bit sloppy but I added support for downloading a specific track with the -t argument.

soundscrape vaughan-1-1 -t psychic-readings-with-fletch

It really is kinda messy, I'm sure there's a better way to handle the "one_track" stuff but this was the first thing that came to mind and it worked... Also changed "if args.group" to "if vargs['group']" cause that seemed more consistent with your code.