Use the new React Context API

[stasm]

React 16.3 is out and it comes with a new Context API replacing the old experimental one.

https://reactjs.org/blog/2018/03/29/react-v-16-3.html
https://reactjs.org/docs/context.html

We should use it in fluent-react. I'd like to understand if there's a way to keep the old one as an alternative for codebases on React 15 or 16.1.

[stasm]

This was fixed by @Gregoor in #406 and released as @fluent/react 0.11 back in January.