Fixed for ExpressJS

[nergmada]

Allow overriding Router

I was trying to follow the example in the HttpServer example to set this up but a few parts kept bugging. Namely that require('router') is not a function. I don't know if this works on other systems and is just a by product of using webpack to package a server, but I've added

config.router so that you can specify the direct expressjs import by doing

import {Router} from 'express';
import hazel from 'hazel-server';
const server = hazel({
//other details
  router: Router
})

It'll still default to the 'router' package if the config.router option isn't specified.

Switch node-fetch for cross-fetch

I know cross-fetch is worse because it's basically a super polyfill for node and browsers without the Fetch API, but node-fetch doesn't seem to work with webpacking either, so I figured that I'd bring a sledgehammer to hammer a nail

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