Include Babel transformation in Webpack loader

[gregberge]

Relative to this issue facebook/create-react-app#3907 (comment).

The goal is to apply Babel transformation directly inside the Webpack loader. Babel will transform code into ES5 with createElement, ES modules have to stay.

We must be able to disable the Babel transformation for advanced cases using an option babel: false.

README must reflect the new change and have a not about this.