Define dev chunk names

Fixes facebook#2046

Author: jstcki

packages/react-scripts/config/webpack.config.dev.js

@@ -71,6 +71,7 @@ module.exports = {
     // served by WebpackDevServer in development. This is the JS bundle
     // containing code from all our entry points, and the Webpack runtime.
     filename: 'static/js/bundle.js',
+    chunkFilename: 'static/js/[name].chunk.js',
     // This is the URL that app is served from. We use "/" in development.
     publicPath: publicPath,
     // Point sourcemap entries to original disk location