getComment(comment, cb) {
      return this._request('GET', `/gists/${this.__id}/comments/${comment}`, null, cb);
   }
Fetch one of the gist's comments
@see https://developer.github.com/v3/gists/comments/#get-a-single-comment
@param {number} comment - the id of the comment
@param {Requestable.callback} [cb] - will receive the comment
@return {Promise} - the Promise for the http request