Feature Request: expose the node to estimators

[noc7c9]

Hiya, first off thanks for the amazing library.

I'm trying to implement a custom estimation that depends on which fields are selected under a type.

Here's an example:

query {
    a: someType { id } # Only id is selected, so calculation happens one way
    b: someType { id, some, fields } # More than id is selected so calculation happens another way
}

I don't believe that the exact behavior I want is achievable with childComplexity unfortunately, I need access to the selectionSet for the given node.

Hopefully that made some sense!

[ivome]

We could just pass the node as part of the estimator args. This would enable a whole bunch of additional functionality and ways to calculate the complexity of a field. Could potentially also be relevant for #41

Happy to merge a PR if you want to add this.