Add compileES6 method so addons can use broccoli-es6-concatenator

There is an issue with es6-module-transpiler that prevents the module being loaded more than once, and this prevents any ember-addons from using broccoli-es6-concatenator. See [this issue](https://github.com/joliss/broccoli-es6-concatenator/issues/20) for a summary of the issue. This PR provides a simple workaround that allows addons to use the instance of the module that's loaded by ember-cli.

I'm reluctant to submit this workaround, but AFAIK there's simply no other solution other than running another instance of broccoli, which is just horrible.
`EmberApp.prototype.compileES6 = compileES6;` should be fine (no need for the function).