lyrics: add 'musixmatch' source

- some more efforts for plugins uniformity : as fetchart, lyrics has now a *sources* option
- *musixmatch* is one of the few sites that does not get caught by the soup scraper ... the perfect candidate for a manual extraction

I put one more function in utils :santa:  to force us to find a name for a top level module with this plugin/unification  code (what about existing `plugins.py`?)
Can this link to the MusixMatch web site for those who don't know what it is? Also, this might be slightly easier to read s a single bullet point in the changelog.