add listenbrainz plugin

[carl-di-ortus]

Listenbrainz support a compatibility for last.fm scrobbler, so a bare minimum adding this is very easy - just change the post URL. It copy-pastes ~300 lines of code. Dirty, but works. Better alternatives are a topic of discussion:

• Add URL field in scrobbler settings (and rename old last.fm plugin to some more generic name like "Audio scrobble"?), this will not allow having multiple scrobblers at once.
• Rename last.fm plugin to generic "Audio scrobble" and introduce a multiple url+username+password sets for handling sumbits. Might save the code duplication, but complicate settings a slight bit.
• Keep plugin separate, and implement listenbrainz API, don't use listenbrainz proxy compatibility. This would still have code duplication on firing/listening events.
• .. maybe more options?

Note: I'm basing this from #257, my original progress and more comments are tracked on forked repo - carl-di-ortus#2

I intend to keep this draft pull request hanging here, just to notify my intention, and gather feedback.