Add Social Media tags

[hanneshauer]

Right now Casper doesn't seem to have any tags that let social media sites such as Facebook or Twitter easily parse the content for their previews; thus sharing Ghost articles requires manual work to provide as much information as possible at a glance. (See this forum post for a user reporting problems with Facebook sharing and pre-filled information)

Implementation of these tags would improve sharing and encourage theme developers to include them as well.

Information on various tags:
Open Graph Protocol (Facebook)
Twitter Cards

[peterblazejewicz]

There could be the reason why default Ghost's theme does not have OpenGraph or Twitter's Cards meta data. Please see foreword to this ticket by @JohnONolan:
TryGhost/Ghost#3064 (comment)
Otherwise it is quite trivial to add (I do this at my work and already wondered about PR to Casper)

[nuclearpengy]

Would be great if this was catered for by default. Having better quality looking 'shares' on social networks is always advantageous.

[jguerin]

Agreed that this would be very nice!

[ErisDS]

This is something we'll be adding to Ghost (#3900), rather than to Casper.