These will be implemented in the following commit(s).

Descriptions are quoted from the MPD Protocol documentation, v0.21.4.
https://web.archive.org/web/20190104200400/https://www.musicpd.org/doc/html/protocol.html

Notes:
- file(name) only has the '==' operator documented, but mpd(1) accepts
  all tag-filter-operators for it.
- AudioFormat only documents '==' and '=~', which is not a regex, but
  accepts '*' wildcards. We don't support this tag type.
- in '(...AND...)', MPD enforces at least two subexpressions, we are
  laxer and allow a single one as well (i.e., just an extra pair of
  parentheses around an expression).
- where possible, exact error messages from MPD 0.23 have been used.

Mopidy can't handle not-matching, so no need to implement this. This is
a seperate commit, so if in the future Mopidy gains this ability, it can
be easily reintroduced.

Note that we support OR-ing multiple base URIS, which MPD doesn't.