TODO after 1.0:

* Fileplayer/Regex tie
. Currently, (file) players are tied to their respective regex and
  knowledge as to what files they can play. 
. The user might want more control there.
. Implement emms-player-alist, associating a regex or a function
  (that will get a track as an argument) with a player.
. This affects emms-player-for in emms.el, and emms-source-file-regex
  in emms-source-file.el

