code: function applyGroupBy(images, options) {
  	return Promise.reduce(options.groupBy, function (images, group) {
  		return Promise.map(images, function (image) {
  			return Promise.resolve(group(image)).then(function (group) {
  				if (group) {
  					image.groups.push(group);
  				}
  				return image;
  			}).catch(function (image) {
  				return image;
  			});
  		});
  	}, images).then(function (images) {
  		return [images, options];
  	});
  }
docs: Apply groupBy functions over collection of exported images.
  @param  {Object} options
  @param  {Array}  images
  @return {Promise}