Support sharing properly (cards meta tags)

[mathieucarbou]

This issue is to support sharing across social media properly by using twitter cards, facebook cards, G+ cards, etc.

Goal is to add some metadata entries in the header that contain the content title, caption, image, etc so that twitter and facebook can read them to generate a sharing card.

Also, links should include UTM tracking attributes.

Here are some details:

• https://moz.com/blog/meta-data-templates-123
• https://dev.twitter.com/cards/overview

[mathieucarbou]

@ljacomet : FYI