Replace SingletonQuery with BooleanQuery on computed singleton field.

Updated use in command parsing and mbsync plugin.
better handling of songs with featuring artists, songs variants versions (live
etc), and songs combinations (lyrics are then appended)
add lyrics plugin unit tests file
update changelog
Update changelog.rst
Allow CLI tests to use :memory: db

This significantly increases performance
Add captureStdout helper and mocks to some tests
Log debug message for configuration location

Helps with issues like #682.
Fix errors in key normalization
Add keyfinder plugin

Closes #662 and #279.
slightly expand docs for keyfinder
keyfinder: handle errors in command execution
Pythonic names for test helpers
Merge branch 'KraYmer-lyrics-enh-tests'
lyrics: minor style/doc cleanup
lyrics is flake8-clean
rewording in changelog
Merge branch 'ktkonrad-singleton'
fix import path to BooleanQuery (#680)
use a lambda for singleton field (#680)

Avoids a naming collision for now. We might revert this later with a decorator
approach.