Add union/interface resolution

[timsuchanek]

To support unions and interfaces, resolution logic is needed to resolve the correct types depending on the node / fragment. For now, we have this logic in the graphql-binding-github which was the first binding that needed this. https://github.com/graphql-binding/graphql-binding-github/blob/master/src/index.ts#L25 We should move this logic to graphql-binding and rethink if it's applied at the right abstraction level.

[timsuchanek]

Implemented in latest beta.