support exclude_tag and exclude_category for /search and /posts

[JulianKingman]

It would be great if I could filter out specific categories or tags. Seems like 'category__not_in' and 'tag__not_in' would do the trick.

[JulianKingman]

@erwstout any chance you can merge the PR? It's about 8 lines of code changed, should be pretty easy to review/approve ;)