[Feature request] Opposite of excluded_file_extensions

[realh]

My music collection has a lot of extra files cluttering it up, so it took me a while to build up the local/excluded_file_extensions option to skip all of them. I thought it would be more useful to have an included_file_extensions option, because a list of typical music file extensions is smaller. That could give you a bit of a problem if both options are used, but you could probably just deprecate and ignore excluded_file_extensions.