cant use babel-preset-stage-0 in js files but you can in jsx files

This is because we have it configured in the generated webpack.config file. We don't have it set up for .js files. Adding it for js files could be troublesome cos it'd mean babel will look at all the js files in the libraries, prob something we want to avoid.

Need to decide how best to support it.

* one option is on name files `.es6`? 
* allow user to configure whitelists?
