Add helper function to convert videos to mp3

This uses variable bitrate encoding, requires LAME
xmonad: ignore xmonadrc files

xmonadrc installs xmonad in a sandbox, at the sake of bypassing
~/.xmonad/xmonad.rc, so I maintain my xmonad.hs file in a different repository
now.

You can get his xmonadrc here: https://github.com/pjones/xmonadrc