Add Transpile Task to Gulp Workflow

[macburgee1]

Whereas there are performance benefits to running ES6 natively in browser, we should provide both ES6 and legacy JS versions of component JS files. In DCF core, we'll keep an ES6 copy of the component's JS file. In DCF Starter, we'll use Gulp to copy the ES6 version to the frontend project. We'll also transpile a legacy version. The directory structure will be as follows:

/
  js/
    dist/
      dcf-lazyload.es6.js
      dccf-lazyload.js
      mycustomjs.es6.js
      mycustomjs.js
   src/
     mycustomjs.es6.js

We'll handle ES6 detection in a follow-up task in DCF core: digitalcampusframework/dcf#302

[macburgee1]

Use the following branch: issue-3-gulp-transpile-task (already pushed to remote)
When ready to convert this task to a PR, run:

hub pull-request -i 3 -h d-c-n:issue-3-gulp-transpile-task -b d-c-n:2.0