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
harmonize all plugins docs

Add a configuration section that lists all options for each plugin.
List options in alphabetic order.
Mention the default value for each option.
Use same sentences to describe concepts common to different plugins eg 'auto
option, install description
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 remote-tracking branch 'upstream/master'
docs: add plugin name in "how to configure" sentences
docs: remove space before colon
fix typos
fetch art doc: restore section title
fix index toc
changelog.rst: remove links to echonest_tempo
Merge remote-tracking branch 'upstream/master'
always a colon between the option name and the rest of the description

plus typos and case change
discogs: Fix #1049: intermittent trailing comma
Plugin docs overhaul tweaks through "duplicates"

Moving alphabetically through the docs. Got a long way to go...