code: function token(len = 20, encoding = 'hex') {
    return randomBytes(len)
      .then((buf) => {
        return buf.toString(encoding);
      });
  }
docs: /*
  Asynchronous generate the token.
  
  @example
  let tokenPromise = yield token();
  
  @param {Number} len, 20 by default
  @param {String} encoding, hex by default
  @return {Promise} 40 length hex string by default