contents: Page.url_format should expose all metadata

I want to add `directory` metadata to each page in `content/pages/` to
place my non-article pages by hand:

  PAGE_URL = '{directory}/{slug}'
  PAGE_SAVE_AS = '{directory}/{slug}/index.html'

To do this, I need the `directory` metadata for formatting the URL.
test_contents: Add tests for metadata export from Page.url_format
Merge pull request #663 from wking/url-format-metadata

contents: Page.url_format should expose all metadata