Added an mpaa_only rename replacement token

The current implementation of the 'mpaa' replacement token includes certifications from around the world (not just MPAA). If the user wishes to limit the values to those used by the MPAA ('G', 'PG', 'PG-13', 'R', 'NC-17' or 'Not Rated') they can use the new replacement token called 'mpaa_only'. The original mpaa replacement token remains unchanged so current users will not need to change their config/settings at all.

I have tested this change for a month on my own system, and it works as intended.