Add optimize-js-plugin to webpack.config.prod.js

Created by: ggregoire

This PR adds the optimize-js lib to the build process:

• https://github.com/nolanlawson/optimize-js
• https://github.com/vigneshshanmugam/optimize-js-plugin

Benchmarks from @nolanlawson:

Browser	Typical speed boost/regression using optimize-js
Chrome 55	20.63%
Edge 14	13.52%
Firefox 50	8.26%
Safari 10	-1.04%