TODO: Get `avconv` working. Oftentimes `play` won't be able to play
      some tracks due to an unsupported format; we'll need to use
      `avconv` to convert them (to WAV).
      (Done!)

TODO: Get `play` working.
      (Done!)

TODO: Get play-next working; probably just act like a shuffle. Will
      need to keep an eye out for the `play` process finishing.
      (Done!)

TODO: Preemptively download and process the next track, while the
      current one is playing, to eliminate the silent time between
      tracks.
      (Done!)

TODO: Delete old tracks! Since we aren't overwriting files, we
      need to manually delete files once we're done with them.
      (Done!)

TODO: Clean up on SIGINT.

TODO: Get library filter path from stdin.
      (Done!)

TODO: Show library tree. Do this AFTER filtering, so that people
      can e.g. see all albums by a specific artist.
      (Done!)

TODO: Ignore .DS_Store.
      (Done!)

TODO: Have a download timeout, somehow.

TODO: Fix the actual group format. Often times we get single-letter
      files being downloaded (which don't exist); I'm guessing that's
      related to folder names (which are just strings, not title-href
      arrays) still being in the group array. (Update: that's defin-
      itely true; 'Saucey Sounds'[0] === 'S', and 'Unofficial'[0]
      === 'U', which are the two "files" it crashes on while playing
      -g 'Jake Chudnow'.)
      (Done!)

TODO: A way to exclude a specific group path.
      (Done!)

TODO: Better argv handling.
      (Done!)

TODO: Option to include a specific path from the source playlist.
      (Done!)

TODO: Make a playlist generator that parses http://billwurtz.com
      instrumentals.html.
      (Done!)

TODO: Make crawl-itunes.js a bit more general, more command-line
      friendly (i.e. don't require editing the script itself), and
      make it use the getHTMLLinks function defined in the new
      crawl-links.js script.
      (Done!)

TODO: Play-in-order track picker.
      (Done!)

TODO: Volume controls. Who knows how to do this? It might have to be an
      argument passed to `play`. Being able to change the volume while it's
      playing would be nice, but I'm not sure if that's really possible.