Include artist country and start/end date in artist info

When looking up artist information on MusicBrainz, their area and start/end dates are also returned. It would be neat to have this information included as well, so you could use this information for path names (e.g., `/path/to/music/Redžepova, Esma (1943–2016)/…`), searches ("show me music performed by people from Uganda born in September"), etc.

This would obsolete the 3rd party plugin [`beets-artistcountry`](https://github.com/agrausem/beets-artistcountry) and it would thus probably be good to reuse the `$artistcountry` variable from that, if it makes sense.

Start by reviewing the beets-artistcountry plugin linked in the issue, especially its $artistcountry variable, and trace how MusicBrainz artist information is incorporated. Define how area and start/end dates should appear in artist metadata, path names, and searches, then verify those uses with the existing behavior.

Written by the indexing model from the issue text.