Add support for link rel="image_src"?

[pdehaan]

Randomly saw this, and surprisingly is/was a thing:

<link rel="image_src" href="http://uncrate.com/p/2016/08/pf-flyer.jpg" />

— via http://uncrate.com/stuff/pf-flyers-x-todd-snyder-grounder/

That site also has the proper open graph tags, so we find the thumbnail anyways, but not sure if we want to add this [here] as yet another fallback.

const imageRules = buildRuleset('image', [
  ['meta[property="og:image:secure_url"]', node => node.element.content],
  ['meta[property="og:image:url"]', node => node.element.content],
  ['meta[property="og:image"]', node => node.element.content],
  ['meta[property="twitter:image"]', node => node.element.content],
  ['meta[name="thumbnail"]', node => node.element.content],
]);

Ref: http://stackoverflow.com/questions/19274463/what-is-link-rel-image-src
Ref: http://www.niallkennedy.com/blog/2009/03/enhanced-social-share.html