Add use_mb_genres config option.

[aereaux]

Description

This is a followup for #3781, adding a config option.

Let me know if I did anything wrong or missed anything, this is my first time adding a config option.

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)

[sampsyo]

Awesome; thanks for doing this! Here is a comment about locating the option in our existing MusicBrainz configuration "section."

[aereaux]

Thanks! Added.

[sampsyo]

Yay; looks perfect! Thank you!!