lyrics: add rest_directory configuration option

[hakkilab]

Description

Fixes #2806

• Adds a rest_directory configuration option to the lyrics plugin that specifies a directory for ReST output, equivalent to the -r, --write-rest command line argument

• Documentation.

• Changelog.

• Tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.79%. Comparing base (60047df) to head (bdd7a46).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6745      +/-   ##
==========================================
+ Coverage   74.52%   74.79%   +0.26%     
==========================================
  Files         162      162              
  Lines       20818    20818              
  Branches     3295     3295              
==========================================
+ Hits        15515    15571      +56     
+ Misses       4547     4489      -58     
- Partials      756      758       +2     

Files with missing lines	Coverage Δ
beetsplug/lyrics.py	90.34% <100.00%> (+10.81%)	⬆️

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[hakkilab]

I added another commit to resolve a failing lint check. Please review this PR at your earliest convenience. This is my first contribution so let me know if any changes are needed and I will update the PR. Thanks!
@sampsyo @snejus