Update isomorphic-fetch, allow fetch implementation to be passed in

[ropwareJB]

https://github.com/nordsimon/graphql-client/pull/10 is favourable if working with node backend and isomorphic-fetch, but it wasn't on my environment.

Allows the fetch implementation to be passed in, as isomorphic-fetch was seemingly ignoring proxy/agent settings.

import GQLClient from "graphql-client"
import HttpsProxyAgent from "https-proxy-agent"
import node_fetch from "node-fetch"

var c = GQLClient({
  url : "http://<host>",
  agent: new HttpsProxyAgent(url.parse("http://127.0.0.1:8080"))
}, node_fetch)