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
Merge pull request #1148 from Kraymer/lyrics_musixmatch

lyrics: add 'musixmatch' source

Conflicts:
	beetsplug/lyrics.py
Privatize global SOURCES in lyrics
fetchart: Privatize global source list (#1148)
Changelog formatting (#1148)