Support DOM References

[JohnDoneth]

Add a constructor to VDOM called Ref that would allow for integrating with third-party libraries that manipulate the DOM on their own. A Ref would allow tracking of DOM elements outside of the DOM patching lifecycle so any changes to those elements would not be overridden and be opaque to the DOM patching process.

Ideally we could keep this code "pure" and not rely on having a real / polyfilled DOM in place for testing and use server-side.