Add generator support for CSS modules (current generator uses old way)

[justin808]

Based on this PR:

shakacode/react-webpack-rails-tutorial#175

We'll stop supporting the old way for the generator probably.

[justin808]

Closes #18. We'll do CSS Modules and the hot reloading as one step for the generators.

[justin808]

We're going to leave CSS configuration out of the generators as this is just changing too fast, and it's not part of the core of ReactOnRails.