request for category unions intersections via URL

[azizpoonawalla]

The functionality for searching a blog by unions and intersections of tags is a powerful feature. The same functionality for categories would also be extremely useful.

[ryan]

This is pretty easy to do if someone wants to tackle it. Add categoryslug_in and categoryslug_and queries that are patterned after the tag equivalents. These should be private query vars. Add a new public query var called "category" patterned after "tag". Convert the existing category_name query to use categoryslug_in.

[johnbillion]

This got fixed somewhere along the line. Worksforme as of 3.1.