title: webpack-dev-server to return auto-generated for specific URL?
body: <p>I have a single page app which connects to a back-end REST API on a different server.</p>
  
  <p>On <code>index.html</code> app load, first thing it does is <code>GET /env.json</code> which contains the URL for the REST API server. Obviously, it is different in prod than in test than in dev.</p>
  
  <p>Is there any way I can add middleware to the webpack-dev-server so that when it sees <code>GET /env.json</code> it serves up automatically generated json (based on env var or other)? </p>
  
  <p>If there is an easier way, I am open to it. My assumption was that dev and test would start a backend server and then configure response to <code>GET /env.json</code>, while prod would have a different file added or dynamically generated.</p>