Support searching through the MPD server

[kimtore]

pms retrieves mpd's database and builds a local search index. Each time a search is performed, the local database is consulted instead of running the search through mpd.

Implementing a way to search directly in mpd might significantly improve usability for Mopidy users, running remote backends such as Spotify or Soundcloud, see #113.

[ephemeralriggs]

The "?" key is unused per default at the moment. Could be a good candidate for this function.

[kimtore]

Agreed.

The search should return results while typing.