Allow to specify a `fetch` implementation via the module API

[FezVrasta]

Right now, we need to pollute the global scope with a fetch polyfill in order to use this module.

It'd be great if the library provided an explicit API to provide the required fetch polyfill, so that an application doesn't need to pollute the global scope in order to make a single library happy.

A possible interface would look like this:

// ES Modules syntax
import Unsplash from 'unsplash-js';
import fetchPolyfill from 'node-fetch';

const unsplash = new Unsplash({
  fetch: fetchPolyfill,
  accessKey: "{APP_ACCESS_KEY}"
});

[EdwardGoomba]

This would be nice - I literally just imported node-fetch in the API module to run it on node.