Add support for top level extensions field.

[riverajo]

The graphQL spec specifies an extensions field in the top level response. It's reserved for implementation specific data. However right now there is no way for a client to reach that data. Some endpoints return useful data about query cost. Perhaps a function like RunWithExtensions and convert Run to just pass nil to it for backwards compatibility and for anyone that doesn't care about them. I'd be willing to put together a PR if this is something that would be integrated.