data: Add access to route metadata in the templating data. It's available under `route`. Also, rename `slug` to `rel_url`, because it's confusing when `slug` could be the route metadata of the same name as provided by the default page source. The old `slug` is still there for backwards compatibility though.

---
config:
    site:
        root: /~john/public/
in:
    pages/about.md: 'URL: {{page.url}}, LINK: {{pcurl("missing")}}'
    pages/_index.md: 'URL: {{page.url}}'
out:
    about.html: 'URL: /%7Ejohn/public/about.html, LINK: /%7Ejohn/public/missing.html'
    index.html: 'URL: /%7Ejohn/public/'