code: function readFile(filename, enc) {
      return new Promise(function (fulfill, reject) {
          fs.readFile(filename, enc || 'utf8', function (err, res) {
              return err ? reject(err) : fulfill(res);
          });
      });
  }
docs: Read the contents of the file filename.
  Asynchronous mode
  @param {string} filename 文件物理路径
  @param {undefined | string} enc 为空返回Buffer类型,'utf8'返回String类型
  @returns {promise}