lastgenre: Add an option to append to existing genres

The current implementation of `lastgenre` ignores any existing genres that a user may have tagged and completely overwrites the genres returned by lastgenre. It would be nice to have the option to append the new genres returned by LastGenre to those that are already available.

### Proposed solution

Extend the plugin to add an append option.

Start by locating the lastgenre plugin and any tests covering genre replacement; inspect how existing genres are currently handled. The change is complete when an append option preserves existing genres while adding the genres returned by LastGenre, with tests covering the new behavior.

Written by the indexing model from the issue text.