listMilestones(options, cb) {
      return this._request('GET', `/repos/${this.__repository}/milestones`, options, cb);
   }
List the milestones for the repository
@see https://developer.github.com/v3/issues/milestones/#list-milestones-for-a-repository
@param {Object} options - filtering options
@param {Requestable.callback} [cb] - will receive the array of milestones
@return {Promise} - the promise for the http request