allow custom request headers to be specified

[zeke]

We have some internal headers that affect how we route requests. It would be nice to be able to specify those somehow, either at request time, or when instantiating the client.

cc @anotherjesse @philandstuff

[mattt]

You can accomplish this by overriding the fetch property of the Client like so:

client.fetch = (url, options) => {
  const headers = new Headers(options && options.headers);

  // Inject the custom header
  headers.append('X-Custom-Header', 'some value');

  return fetch(url, { ...options, headers });
}

I just created #96 to document this in the README.