Tokens for playlist nodes

The default paths for playlists with pathauto end up looking something like this;

http://example.com/stationplaylist/show-name-playlist-2008-05-26

In other words, you've really only got the node title to play with. I'd prefer something like this:

http://example.com/playlist/show-name/2008/05/26

This patch provides tokens to allow this possibility. I'm not quite sure I've implemented it in the best way, but it Works For Me©.