WIP

This actually replaces `{{FEATURES}}` with the contents of the `features` section in `feature.json` ([here](https://github.com/emberjs/ember.js/blob/master/features.json#L2-L16)).  The `defeatureifyConfig` function moves the `features` property to `enabled` [here](https://github.com/emberjs/ember.js/blob/master/Brocfile.js#L35) since `broccoli-defeatureify` requires that format.