allow multiple categories in a "_category" url

If multiple categories are placed in the path of a "_category" URL, separated by "/" characters, the request will return a page listing all pages that contain all of the categories.
using all instead of intersect for test
replacing splitOn with wordsBy, using comma as sep
adding filter links for _category pages
Merge pull request #422 from wcaleb/multi_category_url

allow multiple categories in a "_category" url