Support using FIND_IN_SET to accurately find all posts in a category

Hi, I couldn't see a good way of accurately picking out a single category ID for posts that are in many categories, for example to see all posts that have stats in the Uncategorized category but not *only* that category (like equals would do). Possibly "includes in set" is not the best label, but I tried to pick one that roughly matched the MySQL function.

So this adds the ability to use the MySQL FIND_IN_SET function to the list of filters.