feat: Add genres config option to write main genre, sub-genre, or both

[Samik081]

Adds a genres config option (sub | main | both, default sub) controlling which Beatport genre fields are written to beets' multi-value genres field.

• sub (default): sub-genre, falling back to the main genre — exactly the current behavior
• main: always the main genre
• both: main genre first, then sub-genre

BeatportTrack now keeps genre and sub_genre as separate fields; the selection happens in the plugin (_get_genres()), validated via confuse as_choice, so an invalid value fails loudly instead of silently mistagging.

Verified end-to-end against the live Beatport API with track 16772903 from the issue (Trance (Main Floor) / Hard Trance): track_for_id returns both genres in order, a real singleton beet import --search-id writes genres: Trance (Main Floor); Hard Trance to the library, and the file itself carries both values as multi-value genre tags. The default mode was also live-verified to keep today's behavior.

Fixes #37