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
Add a 'fallback' option to facilitate working around the 100 queries/day google
limit by marking files as 'visited' so they are not considered for lyrics search
on the next beet run.
I've put my own google_engine_ID as default value in the code but could be
reconsidered, this engine contains databases known to be scrappable by the
plugin algorithm though.
