Add Slug Template Function

This adds a slug template function. It allows you to make url slugs for articles, posts, etc

www.example.com/articles/12/slug-title-goes-here

The function currently doesn't support any non-ascii characters. One possibility is to map accented characters to their non-accented ascii character. E.g. Ä => A

I'm not sure whether the function should be available outside of templates.