Adding support for restricting category read access based on user groups

Added this functionality since it's a core feature for us as we are evaluating using NodeBB for a game forum. The most common use case:

Setup read access restrictions to a category in the forum based on user groups. i.e. if a user is part of a "test group" he will be able to access the "Testing Feedback" category. If the user is not part of the "test group" (or is anonymous) then access is denied for him. 

This first contribution accounts for read access only. I'm planning on adding write access next.