Negative segment num to return url with segments removed from end

This addition to Segment is in response to important yet missing Statamic core functionality of traversing up a url path. I will also be submitting a pull request for an alternative syntax shortly so you can decide if you prefer one of the two:

    {{ segment num="-1" url="/blog/categories/whiskey" }}
    Outputs: /blog/categories

    {{ segment:parent_url }} or {{ segment num="-1 }}
    Outputs: /blog/categories