Add ES6 Support about wpkickstart

[aubreypwd]

@efuller getting you involved here as promised!

So here are my initial thoughts:

I want whatever this is to simply make an ES5 version of my ES6 script e.g. I make services/my-service/assets/js/my-script.js and it's written in ES6. The thing we build finds that ES6 script (automagically, maybe it has an @es6 tag in the filedoc maybe there's a thing out there that can detect an ES6 script?) and makes a services/my-service/assets/js/es5/my-script.js file I can enqueue safely.

I'm thinking that simple. That way I can write in whatever I want and our tool here will just compile es5 versions of the code for us to use. Open to discussing this further.