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.

---
admin: true
url: /
in:
    pages/one.html: ''
    posts/2016-01-01_post1.html: ''
out_contains: |
    1 pages