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:

• [x] support an origin option (#180)
• [x] allow the user to add their own custom values in front matter (part of #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 #1083 also have treated a head/header/footer value passed as front-matter? (see #1255).