Hot reloading support

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Hot Reloading of React code is very powerful when done correctly, and we should support it.

The best way to do so seems to be React-Hot-Reloader version 3, currently not released : gaearon/react-hot-loader#240
Gaearon's take on why it is not yet released : gaearon/react-hot-boilerplate#97
TL;DR: is is often misused and badly integrated, thus people end up with bad production builds. He is now working on bringing hot reloading directly into React.

With rails/webpacker#55 the first step for the support has been done, now we need to document how to enable hot reloading (mostly changes to Webpack config and the root component). An idea would be to include the boilerplate to enable hot reloading into our JS code : we render the HR components, then call the app root component in them, so the user does not need to change its code to use hot reloading, only enable it in an initializer or a react_component call.

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Implemented by #7