Pipe the output of this writer to a file.
function save(file, options) {
  var stream = fs.createWriteStream(file, options);
  function onSave() {
    stream.removeAllListeners();
    this.emit('save');
  }
  function onError(err) {
    stream.removeAllListeners();
    this.emit('error', err);
  }
  stream.on('finish', onSave.bind(this));
  stream.on('error', onError.bind(this));
  this.pipe(stream);
  return this;
}