code: function(url, filepath, fn){
          var output = fs.createWriteStream(filepath)
          this.request
              .get(url)
              .on('error', function(e) {
                  fn(e)
              })
              .pipe(output)
              output.on('close', fn)
      }
docs: 下载
  @param   {String}   url      下载地址
  @param   {String}   filepath 写入地址
  @param   {Function} fn       回调函数