Add a lyrics backend that scrapes results from google custom search api

This PR extends the lyrics plugin behaviour by adding a 'google' backend.

Unlike existing backends that scrap pages using dedicated regexes, the pages returned by the google search are screen-scraped *generically* using BeautifulSoup to return the longest paragraph of the page. Some heuristics are then applied to remove false positives.

The doc has been updated with infos about the new config options.