Added an mpaa_only rename replacement token

The mpaa replacement token includes certifications from around the world. If the user wishes to limit the values to one of 'G', 'PG', 'PG-13', 'R', 'NC-17' or 'Not Rated' they can use the added mpaa_only replacement token. The original mpaa replacement token remains unchanged.
Merge pull request #3166 from harrv/develop

Added an mpaa_only rename replacement token