harmonize all plugins docs

original issue #1012
 
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
It might be helpful to add one element to this standard "installation" preamble: mentioning the name of the plugin again. This way, it's as easy as possible to know exactly what to type into your config file.

> To use the ``beatport`` plugin, first enable it...