/**
 * flatten an object means each keys are a jsonpath.
 * jsperf: https://jsperf.com/talend-flatten
 * @param {object} obj the source object
 * @param {object} options
 * @return {object} flatten object
 * @example
 * flatten({ level1: { level2: 'foo' }})
 * // { 'level1.level2': 'foo' }
 */
export default function flatten(obj: object, options: object): object;