Load environment file in development

Created by: ayrton

Adds support to load environment variables via a .env file for development using the very stable and battle tested ⚔ npm package dotenv. For more context see https://github.com/facebookincubator/create-react-app/issues/687.

I've tested this out by doing the following:

npm run create-react-app my-app
cd my-app
echo "NODE_PATH=." >> .env
npm start

In my-app/src/index.js I changed import App from './App'; to import App from 'src/App'; and verified all still works (including tests)