webpack CSS HMR bug

1. npm install
2. npm start
3. Visit http://localhost:8080/
4. Open index.css and change the background-color

Expected: The CSS is hot reloaded.

Actual: The whole page is reloaded.

See index.css for further comments.

Disable css-modules in webpack.config.js to see how well CSS HMR works when css-modules is disabled.

Check out the fix branch for a fix!