How to handle npm modules which include es6?
I wonder what's the appropriate way to configure skpm in a way, so ES6 in `/node_modules/` will be transpiled to ES5 by babel.

Right now I'm trying to use `sketch-polyfill-fetch` (inside Mocha context), but I got the following exception
`SyntaxError: Unexpected keyword 'const'. Const declarations are not supported in strict mode..`
And after that `cocoascript-class` also uses `const` declarations.

Of course, I can mutate webpack via `webpack.skpm.config.js` and update "exclude" options, but I feel like I'm doing something wrong here.