allow multiple categories in a "_category" url

This implements a feature that @lmullen [requested last year on the gitit-discuss list](https://groups.google.com/forum/#!msg/gitit-discuss/5A8PUbZ1gC4/_KcgeXgLQ4oJ).

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.

Since this is my first real foray into the Gitit source code, extra scrutiny is warranted, but limited testing on my personal install suggests that it works well.

I copied the "splitOn" function from Framework.hs because as a Haskell newbie I'm not sure whether exporting splitOn from there (and then importing it here) would break something else.