Allow WP_Comment_Query::query to filter by category

[sambauers]

The attached patch allows WP_Comment_Query::query to accept three additional arguments:

• cat
• categoryin
• categorynot_in

The resulting comments are then filtered by these category arguments as well as any other arguments.

These arguments work the same way as their WP_Query counterparts.

I would appreciate advice on the appropriateness of using INNER JOIN (as I have done here) to join in the taxonomy tables to the query.