Allow to replace default execute function

[IvanGoncharov]

This PR allows providing your own execute function instead of the default one from the graphql-js.
It’s a small change that adds a lot of flexibility since you can wrap standard execute and it opens new possibilities for middlewares.

Personally, I need it to proxy calls to 3rd-party APIs.
It’s an alternative to #253 but introduces less code and more flexible mechanism.
It’s very important for us as it will be the last change required to switch graphql-faker from custom forks of graphql-js and graphql-express.
@wincent @leebyron Can you please review it?

[matthewerwin]

@IvanGoncharov why has the signature been changed to a single args object? Public API that you cited on my PR http://graphql.org/graphql-js/execution/#execute doesn't take a single object argument so this seems like this code isn't working when the override execute isn't passed in.

[IvanGoncharov]

@matthewerwin execute was extended to also support object as argument see here:
graphql/graphql-js#867
But no one updated the official docs 😞

Also, my PR to exposing ExecutionArgs type was merged: graphql/graphql-js#988
So it's easy to do type checks in Flow and TS.

I think receiving/returning objects is better than 7 separate values.

[matthewerwin]

@IvanGoncharov right on. Maybe make the update to the docs on that part of this PR. Thanks for the comprehensive info

[matthewerwin]

@IvanGoncharov I think before this PR gets merged it would be great to add two things:

1. The documentation updates.
2. change "response.statusCode = 400;" to "response.statusCode = contextError.status||400;"

First one to ensure everyone understands the execute capability, the second so you can send back meaningful codes (e.g. 401 for unauthorized).

[IvanGoncharov]

@leebyron Can you please review this PR?