Make it easy to profile build

http://chrisbateman.github.io/webpack-visualizer/

To generate the visualizer you currently need to perform this command:

```
NODE_ENV=production ./node_modules/webpack/bin/webpack.js --config node_modules/@volst/react-scripts/config/webpack.config
.prod.js --profile --json > stats.json
```

Would be nice if this is an integrated command.