bpd: support MPD stickers

The MPD protocol has a "stickers" feature that is conceptually equivalent to beets' flexible fields. In the real MPD it's backed by a standalone database, and song ratings is suggested as a possible use case. I'm yet to find an MPD client in the wild that uses this feature so it's not a high priority.

For implementation I think it would be straightforward to make stickers map to flexible fields. The question is whether to namespace it (e.g. a sticker called `rating` is stored in beets' database as the field `sticker_rating` or similar) or expose all (some?) of beets' (flexible) fields as stickers. It depends a bit on use cases in clients, I imagine.