lyrics: add musixmatch source
lyrics: add 'sources' option
lyrics: preserve default order of sources
move helper functions from utils to plugins
add a link to musixmatch.com
lyrics: update unit tests
remove test_util.py
remove print
lyrics: Connect force option to CLI (#1150)
Privatize global SOURCES in lyrics
fetchart: Privatize global source list (#1148)
Changelog formatting (#1148)
Added an interface for plugins to filter imported files
Did some changes to use the latest beets source with the enumerate-only option and the file filter.
Some changes for the code structure
Some changes to meet the flake8 rules
Testcases failed because one of the new tests fiddled around with the plugins singleton. Fixed this.