beets: Add the external beets-alternatives plugin

It's not included in upstream beets but are linked in the documentation
under "Other plugins", see:

http://beets.readthedocs.org/en/v1.3.15/plugins/index.html#other-plugins



I found this one particularly useful for syncing files to varios media
players that refuse to read my FLAC files properly.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>