Differential serving example.

This repo is a step-by-step demonstration of how to implement differential serving on a toy app. Each step is conveniently marked with the following git tags:

• step-0: Start.
• step-1: Update babel config to add clientModern environment.
• step-2: Create a common config object.
• step-3: Add webpack client-modern webpack config (this is the big part).
• step-4: Add module/nomodule code in src/server/index.js.