Update examples in docs to reflect new UI enhancements.
Add Discogs and general source weight penalty (configurable, undocumented).

Discogs (and other individual plugins) should return a penalty between
0.0 and 0.1, which will be multiplied by the general source weight. This
allows individual plugins to be weighted against relative to each other,
and amplified together against other penalty categories (track, title,
etc.)