Added the list command

This command will let you search the database for metadata. For example, if you
just wanted all the artists in your database, you can use `list artist`. If you
wanted to find all albums that the band "name" owns, you would use:
`list album artist "name"`.

You could search for `artist`, `album`, `title`, `track`, `name`, `genre`,
`date`, `composer`, `performer`, `comment`, or `disc`.

For more info, see:
[The wiki](http://mpd.wikia.com/wiki/MusicPlayerDaemonCommands#list).