flake8 cleaning beetsplug/*

Remaining warnings are related to visual indentation.
Merge branch 'KraYmer-flake8'
restore some comments from #684

I wanted to leave a few of these as comments -- they serve more as separators
than as internal documentation.
fix up some indentation warnings in plugins
flake8-cleanliness in missing

This is a little bit dumb, but one way to get the style checker to accept the
nice alignment in this plugin is to make it a dict.