Adds M3U playlist support as a query to beets and thus partially
resolves issue #123. The implementation is heavily based on #2380 by
Robin McCorkell.
It supports referencing playlists by absolute path:
$ beet ls playlist:/path/to/someplaylist.m3u
It also supports referencing playlists by name. The playlist is then
seached in the playlist_dir and the ".m3u" extension is appended to the
name:
$ beet ls playlist:anotherplaylist
The configuration for the plugin looks like this:
playlist:
relative_to: library
playlist_dir: /path/to/playlists
The relative_to option specifies how relative paths in playlists are
handled. By default, paths are relative to the "library" directory. It
also possible to make them relative to the "playlist" or set the option
or set it to a fixed path.
