lyrics: google backend should turn up (even) more results 

I spent one hour attacking my *Latin* folder with lyrics plugin :  
- added 2 new sources websites to the default beets Google Custom Engine, noticeably maxilyrics.com seems to have a good portion of latin lyrics
- tweaked the plugin code to find more results by skipping text between parentheses when comparing titles and dissociating valid lyrics containing lyrics credits from invalid lyrics text placeholder