adding es6 module transpiler filter

Hi all,

This PR was suggested to me by the friendly Ember folks over at Yapp.  The filter uses Square's ES6 Module Transpiler (https://github.com/square/es6-module-transpiler) through a Ruby gem I created (https://github.com/izhan/ruby_es6_module_transpiler).  The gem is modeled closely after a Rails gem (https://github.com/izhan/es6_module_transpiler-rails) and will continue to be improved in the future.  For an example of the filter in action, see https://github.com/izhan/rake-pipeline-tester.

I am open to suggestions for improving both this PR as well as the Ruby gem.

Thanks