Add lastimport plugin by @rafi

From rafi/beets-lastimport at 72c199d599ee76f2a5fa366a032ad33491f2c82c.
Safer GET parameter formulation for AS URL
Style fixes
Use MatchQuery for mb_trackid lookup
Use shared Last.fm API key
lastimport (#1009) docs: translate from README
Tidy lastimport docs (#1009)
lastimport: Declare play_count type

Now you can get the fanciness of the numeric field without configuring the
`types` plugin.
Merge branch 'lastimport'
lastimport: Add defaults to docs