feat: support passing custom fetch into GraphQLClient constructor

[tarekrached]

Addresses #40, needs documentation and tests.

Use like so:

import { GraphQLClient } from 'graphql-request'

const client = new GraphQLClient(endpoint, { fetch: myCustomFetch })

[saurabh-codeword]

I use this library in node, and this feature will make it easier.

currently I have to do this:

import fetch from 'node-fetch'

// @ts-ignore
if (!global.fetch) {
  // @ts-ignore
  global.fetch = fetch
}

[jasonkuhrt]

@saurabh-codeword You shouldn't have to do that anymore.

[jasonkuhrt]

@tarekrached this doesn't address the aspect of #40 which is about bundle size.

But anyways let's get this merged.

Yes please add docs and tests.

[fischerman]

This PR is a great addition to enable writing API tests since you can have multiple cookie jars.

[tarekrached]

Heads-up - I'm pretty slammed at my day job, so I'm having trouble finding time to add the tests and documentation. If someone wants to pick this up, please go for it! (We're using an internal build off this branch so we are unblocked.)

[tarekrached]

superseded by #212