Implement mirrored values inside namespaces declared as hashes.

This is a new incarnation of the outdated pull request #15 of @lehni - this time with an example in the README and specs.

It shouldn't break any existing code since it's very unlikely anybody used symbols as meta tag values until now. But it DRYes up quite a bit of duplication on open graphs and twitter card declarations of my apps.

Thanks for considering this little feature!