code: function addFavicons() {
    var stream = new Transform({objectMode: true, highWaterMark: 2});
  
    var hosts = {};
    var downloads = [];
    stream._transform = function (page, _, callback) {
      stream.push(page);
      var host = url.parse(page.url).host;
      if (!hosts[host]) {
        downloads.push(hosts[host] = download(url.resolve(page.url, '/favicon.ico')).then(function (result) {
          if (result.statusCode === 200 &&
              !(result.headers['content-type'] &&
                result.headers['content-type'].indexOf('html') !== -1)) {
            stream.push(result);
          }
        }, function () {
          //ignore errors downloading favicons
        }));
      }
      callback(null);
    };
    stream._flush = function (callback) {
      Promise.all(downloads).done(function () {
        callback();
      }, function (err) {
        stream.emit('error', err);
        callback();
      });
    };
    
    return stream;
  }
docs: Look for favicons and add them to the stream if they exist
  
  @returns {TransformStream}