blog: linking blogpost on facebook is using authors icon as article image

[krzysztof]

Result:

We should set open graph image property on each blogpost to Zenodo logo by default and optionally to a custom image: https://developers.facebook.com/docs/sharing/webmasters

[krzysztof]

This PR #34
fixes it up for facebook:

But still doesn't work for twitter (URL is not parsed)

[krzysztof]

twitter also works now, but requires generating a better description from the blogpost