feat(postgraphql) Add EventListener object to Context object. (#501)

[Belline]

This implements the solution described in #501 , allowing Plugins to reach the req/res objects to set Cookies and other uses as well

[benjie]

Great PR: small, clean and simple. However, I think you could use:

#601

To achieve the same goal without requiring any code changes?

[chadfurman]

@benjie we need access to the response object, as well. #601 only provides the request object

[benjie]

You should be able to get res from req.res but feel free to open a PR adding res to #601

[chadfurman]

TIL we can get res from req

[fictorial]

I don't see how req.res is available since this call seems to happen before the graphql is executed.

I don't see a way to set a cookie or other header from within the current flow. It appears that postgraphile as a lib doesn't expect to be middleware and thus does not call any next function it is given so that option is out.

Is there a standard way to hook into the flow to be able to set a response header based on the graphql response?