Alternative syntax for using negative num values to get segments from end of url.

I think this aligns better with the current api than my previous pull request and allows people to get single segments in reverse instead of `{segment}` returning the url.

This addition to Segment is in response to important yet missing Statamic core functionality of traversing up a url path.

    {{ segment num="-1" url="/blog/categories/whiskey" }}
    Outputs: categories

    {{ segment:url num="-1" url="/blog/categories/whiskey" }}
    Outputs: /blog/categories