support open graph (exploration)

[Fil]

This branch (which is probably not going to be a PR) explores what's needed to support open graph (#178) and rel-canonical (#179). It raises a few questions:

• support an origin option (An origin option for features that need absolute URLs #180)
• allow the user to add their own custom values in front matter (part of Expose front matter (meta data) to the page? #1036)
• ["meta[property='og:image']", "content"] must be an absolute URL, which means we need a proper way either to apply the origin+base transform after the HTML has been produced, or conversely that we might want to process a file with an absolute url that corresponds to the site as fileattachments? (There is also og:image:secure_url)
• Should rewrite head, header, footer #1083 also have treated a head/header/footer value passed as front-matter? (see head, header and footer can be specified as a function #1255).