Added commands to interact with stored playlists.

Added support for following MPD commands:

| command      | protocol spec |
| ----------------    | -------------------- |
| update           | http://www.musicpd.org/doc/protocol/ch03s06.html#command_update |
| listplaylists    | http://www.musicpd.org/doc/protocol/ch03s05.html#command_listplaylists |
| listplaylistinfo | http://www.musicpd.org/doc/protocol/ch03s05.html#command_listplaylistinfo |
| load               | http://www.musicpd.org/doc/protocol/ch03s05.html#command_load |
| playlistadd      | http://www.musicpd.org/doc/protocol/ch03s05.html#command_playlistadd |
| playlistclear    | http://www.musicpd.org/doc/protocol/ch03s05.html#command_playlistclear |
| playlistdelete  | http://www.musicpd.org/doc/protocol/ch03s05.html#command_playlistdelete |
| playlistmove  | http://www.musicpd.org/doc/protocol/ch03s05.html#command_playlistmove |
| rename         | http://www.musicpd.org/doc/protocol/ch03s05.html#command_rename |
| rm                | http://www.musicpd.org/doc/protocol/ch03s05.html#command_rm |
| save             | http://www.musicpd.org/doc/protocol/ch03s05.html#command_save