Provide a way to add the '.js' file extension to the end of module specifiers

In order to use es6 modules in the browser, you need a .js file extension. However output doesn't add it.

In ts:
import { ModalBackground } from './ModalBackground';
In ES2015 output:
import { ModalBackground } from './ModalBackground';

Ideally I would like this to be output
import { ModalBackground } from './ModalBackground.js';

That way I can use the output in Chrome 51

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Webpack boilerplate</title>
  <script type="module" src="index.js"></script>
</head>
<body></body>
</html>

Related to #13422