Refining the setting of site_tag when twitter:creator is present.

This is an attempt to close #935. I added a class method that
will take in the current value for site_tag and og_tags and
return a new value for site_tag if twitter:creator is set in
og_tags. If nothing is set, it returns the original value of
site_tag.
I was trying to duplicate the previous behavior, which was that this function was being called right after this filter (which is to say, right after site_tag()). Of course, your objection is right on and it should be set to a higher priority to allow it to be overridden. I'll update the PR.