Allow use of `url` or `slug` page variable

[kdheepak]

Other static site generators tend to support the use of a url or a slug variable, which is the route the final page is served at.

Currently, I wish to have a filename called 00-intro.md and have the url be introduction-to-julia or introduction-to-julia.html, if inside 00-intro.md there is a page variable that contains a valid url.

[kdheepak]

Minor improvement suggestion: it would be nice if the slug meta variable that is returned by pagevar always contained the final url. Currently, it contains the final url if it is set by the user or it is nothing, and the user has to check that and then parse the filename to figure out the final url.

[tlienart]

Yes that makes sense, I'll add another issue for that