Feature: Addons add commands to the CLI

You can now have your addon implement a `includedCommands` hook which returns an object that will be merged with ember-cli's default commands and passed into the Command.extend method for you.
specifically [cli-test](https://github.com/stefanpenner/ember-cli/blob/master/tests/unit/cli/cli-test.js) and [help-test](https://github.com/stefanpenner/ember-cli/blob/master/tests/unit/commands/help-test.js) both have it manually defined in a few places. That's what I was thinking though, I thought I saw that somewhere.