Feature/es6 module build

Created by: neeharv

HACK ALERT. This is an extremely WIP hacky way to do split builds into ES6 and ES5 builds, as described in https://github.com/facebookincubator/create-react-app/issues/3125. I am simply putting this up to spark discussion on the right way to do this.

This works by -

1. Turning webpack.prod.js into a factory function that takes in target and returns different configs with varying targets
2. We suffix the module build assets with .es6.js (this can be moved to config easily)
3. For the ES6 build, we don't include polyfills
4. We get 2 asset-manifests this way, and hence 2 different service workers that load the relevant built code
5. Switch over to "uglifyjs-webpack-plugin": "^1.0.0-beta.2" so we can minify the ES6 code
6. It currently only spits out the ES6 build only in this PR, because the HtmlWebpackPlugin looks at webpack's stats file to generate the variables for the template. I have a local version of HtmlWebpackPlugin that reads the assets of both builds and makes them available for the template, but that is beyond the scope of this and an extremely bad way of doing things

An alternate to this would be to somehow get webpack to accept two different loader configurations for two different entry points, in the same build. AFAIK, this is what @geelen was trying and webpack today doesn't support this. Hence the above approach of running two parallel builds and then using a custom templating job to add the asset names of the ES5 and the ES6 code to the output html.