Fixed up cmus script

Went nuts on this one.  It's 20% smaller than the rhythmbox version, documents its cli args, and is more flexible.

docopts does most of the legwork.  Their bash examples left a lot to be desired, but the actual functionality was great.  They let me write documentation, take args, and set good defaults all in one fell swoop.

This script still reads your music dir and sends the results to cmus.  It gained flexibility from the `--list` option, which takes a tag.  Tag can be genre, artist, album, or song.  I used find with min and maxdepth to only find the directories corresponding to one of these tags, which helped performance tremendously.

But wait, what if someone isn't using Music/genre/artist/album/song?  Well, there's a command to define the structure that gets assumed.  genre/year/artist/album/song would work too.  The only real requirement is that your songs are normalized.  (Also note that song is treated a little differently since it's a file not a dir and since some double cds have album/cd{1,2} so I couldn't know the depth past album.)

The other fun option is `--random`.  It takes whatever you're listing and picks randomly instead of displaying a dmenu.  I don't know that I'd ever want to play a random genre, but I like that I found the flexibility.