allow import "path/style.css" inside jsx (update webpack.config.js bo…

[DivadNojnarg]

This will allow:

import "path-to-css/style.min.css";

inside any srcjs/file.jsx without crashing yarn run webpack. Maybe there was a reason not to put it. Let me know ;)

[timelyportfolio]

@DivadNojnarg I am good with this change. Then hopefully you can eliminate the override section in your book. I'll plan to merge this along with #49 and #45, update React, and then submit to CRAN over the next couple of days.