ES6 Support

[ericelliott]

Since Babel has good support for many of the most useful ES6 features, and Babel is capable of working in an online REPL, we should add ES6 support to GitBook exercises.

Using Babel will also provide built-in support for JSX, so it will be possible to create exercises for React.

[danecando]

:+1:

This would have been so perfect for my current requirement :)