logging: make levels/formats configurable

[9999years]

Description

This keeps the root logger from swallowing logs, for example logs emitted by plugins and dependencies (such as the MusicBrainz API client).

To Do

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

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.72%. Comparing base (5beb3d1) to head (071b4e1).

Files with missing lines	Patch %	Lines
beets/ui/__init__.py	70.00%	8 Missing and 1 partial ⚠️

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