build/lib/util.ts/filter
export function filter(fn: (data: any) => boolean): FilterStream {
	const result = <FilterStream><any>es.through(function (data) {
		if (fn(data)) {
			this.emit('data', data);
		} else {
			result.restore.push(data);
		}
	});

	result.restore = es.through();
	return result;
}