Add opengraph support

[ssyberg]

Would be great to allow for opengraph field support here as well!

[Elliot67]

Would also be great to have json-ld support.

[jacobsfletch]

For now you can define your own custom og or json-ld fields using the fields property (fixed in PR #48). Long-term, I do think this is a good idea and we should definitely come up with some permanent solution for this. I'm going to migrate this to a discussion with the "Ideas" category so that we don't lose sight of this feature.