Provide addon's own public build time API for enabling/disabling deprecations

[rwjblue]

The internal usage of underscored EmberENV properties for toggling deprecated features should not be made to seem as public API (we currently mention in our README).

Instead, we should provide our own build time API to allow toggling. Something like:

new EmberApp(defaults, {
  'ember-2-legacy': {
    'underscore-actions-support': false
  }
})

The default for each item would be true, and you'd override with false to disable a given legacy feature.

[thoov]

Working on this now