This patch allows the client to load a playlist file from the playlist
directory with a plugin.  This can be used with the "load" command,
but the client has to pass the file name including the suffix.  We
will probably use the music directory in the future, to support
playlist files inside the music directory.

This patch integrates the playlist plugin API to the MPD core.  We'll
be able to do much more in the future with that API, that's just the
beginning.