Context and Root

[lucasconstantino]

I see this interface is missing the possibility of adding a context or a root to the query execution. Would you be interested in a pull-request?

[mstn]

Hi sorry again for the late reply. I would accept PRs.

But, I was going to review this package at the light of Apollo 2. Perhaps, it makes sense to think about it in the context of Apollo 2. @mlabrum has done something in this direction.

See #4