Add support for fediverse:creator

I've tried to do the initial integration, mainly looking at other tags how they were done.
If I install the OpenGraph submodule I got the new field. But not sure if anything special needs to be done to update existing installations and if there is need for extra test for new field.

Thank you for putting that together.

Please create the merge request from your issue fork and we can see how it works in the tests.

Merge request created. It took a bit of trial and error (and help from Daniel Pickering) to find how to do that. :)