Consider using esri-loader?

I like how you've [isolated use of the JSAPI to middleware](https://github.com/Esri/react-redux-js4/search?utf8=%E2%9C%93&q=%22from+%27esri%22&type=).

It seems like that pattern would benefit from using [esri-loader](https://github.com/Esri/esri-loader) rather than [treating esri modules as externals](https://github.com/Esri/react-redux-js4/blob/19569e0bdcbf9b15c3dfc491094192806c3ba241/webpack.config.babel.js#L24) and then [adding the JSAPI to the page via script tag](https://github.com/Esri/react-redux-js4/blob/19569e0bdcbf9b15c3dfc491094192806c3ba241/index.html#L10) and using it to [load your bundle as an AMD module](https://github.com/Esri/react-redux-js4/blob/19569e0bdcbf9b15c3dfc491094192806c3ba241/webpack.config.babel.js#L17). Using esri-loader should improve initial load times (especially on mobile) and would allow users of this boilerplate to adapt it for isomorphic/universal (server-side) rendering techniques if their app calls for it. You can read more about those benefits and how it works [here](https://github.com/Esri/esri-loader/blob/master/README.md#why-is-this-needed).

Would you be open to a pull request that switches over to using esri-loader?