lastgenre: Multiple (concatenated) whitelists

It would be really nice to be able to not just replace the the genre list supplied, but to be able to append a list to the default list that is gathered from wikipedia. The wikipedia list excludes many religious genres and doesn't have a genre for "Holiday" or "Christmas" or any other particular holiday genre. I would like to be able to add these by having a separate file in my user config that is appended to the default generated list.

Start at the lastgenre plugin and trace how its default genre list is generated from Wikipedia and how the user config currently replaces it. Done means a separate user-configured list can append entries such as Holiday or Christmas to the generated defaults without removing the existing replacement behavior.

Written by the indexing model from the issue text.