Support open graph meta tags

[wligtenberg]

To have a nicer presence on twitter I think it is good to also support the open graph meta tags.
I tried but up until now I have not yet succeeded in adding them to your template.

https://github.com/whiskyechobravo/pelican-open_graph/tree/9101b3333c9c69490b9943bf21f608491f2cfeea

Any help would be greatly appreciated!