Add site.page(path) and site.document(path)

[ia3andy]

This will allow to find pages by id. This way user won't have to resolve any url by themselves
For example:

{site.page('rss.xml').url}
{site.page('/pages/about.html').url}

{site.document('_posts/my-post.md').url}

We should also remove the rootUrl from site, it is available from the site.url.root if really needed.