Implement features from jekyll-seo-tag

[fititnt]

Check a few features from https://github.com/jekyll/jekyll-seo-tag. We use it at webiwg/webiwg-issues#11. It is whitelisted for github pages. If does not add jekyll-seo-tag, I guess that would be nice to at least add hese twitter cards and facebook stuff.

Also it make use of some _data/* and organize ident the code a little different, like

facebook:
  app_id: 1234
  publisher: 1234
  admins: 1234

instead of

facebook_app_id: 1234
facebook_publisher: 1234
facebook_admins: 1234

I'm not one expert on jekyll/ruby, so maybe my suggestions now are not very good

[woliveiras]

Great!

I use like this: https://github.com/woliveiras/__s/blob/master/_includes/head.html

But jekyll-seo-tag is cool!