add set/clear key for cookie

This would reference issue/request:
https://github.com/hapijs/hapi-auth-cookie/issues/26

Unless I was doing it wrong however in using the original implementation I could never set request.auth.artifacts and retain that through the session lifecycle.

With this change though you can manipulate the data and add/remove at will. Personally I've been using this for validation tokens when posting data and clearing it out after to prevent spam.