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
ftintitle: add 'auto' option
restoring ft_in_title implementation
register import hook only if needed
restore write argument for ft_in_title
move metadata management in command function
Added "ifempty" config to embedart plugin, re: issue 1020
Whoops, take two.
Slight style tweaks.
Whoops, trailing space. Take 2.
Seperated some code out from extract() into get_art(item). Cleaner and quicker way to check for embedded art.
Working on issue #1028, resize images only once per album.
Add tests for #1029

The mbngs library does not like to be called with whitespace-only criteria.
Fix #1029: whitespace-only searches
Merge commit 'ca431b0' (part of #1033)

This is the part of PR #1033 that fixes #1020.
Minor fixes for #1033