feature request: add `readpicture` support

[DanceMore]

HomeAssistant mpd client supports album art and I wanted to know how that worked. tcpdump of port 6600 showed readpicture calls on the wire before headers like:

type: image/jpeg
binary: 1837

followed by obvious binary blob that's probably an album art.

upstream docs: https://www.musicpd.org/doc/libmpdclient/readpicture_8h.html