Addon configuration.

* Replaces #1914.
* Adds basic unit tests for `updateCache` method.
* Add `Project.prototype.configPath`.
* Add `Project.prototype.getAddonConfig`.
* Update `Project.prototype.config` to merge addon config with application config.
  * Ensure that app config ALWAYS wins.
  * If no app config exists, return addon config result.
Refactored this to `Project.prototype.config` (and removed this `configureAddons` function). If you need the configuration in an addon's `included` hook, call `this.project.config(this.app.env)`.