getAddonsConfig(env, appConfig) {
    this.initializeAddons();

    let initialConfig = merge({}, appConfig);

    return this.addons.reduce((config, addon) => {
      if (addon.config) {
        merge(config, addon.config(env, config));
      }

      return config;
    }, initialConfig);
  }
Returns the addons configuration.

    @private
    @method getAddonsConfig
    @param  {String} env       Environment name
    @param  {Object} appConfig Application configuration
    @return {Object}           Merged configuration of all addons