function transform(options) {
    return through2.obj(function(file, enc, next) {
      var self = this;
      renderContent(file.contents.toString('utf8'), options)
      .then(function(output) {
        file.contents = new Buffer(output);
        self.push(file);
        next();
      });
    });
  }
Same as writeFile but works with streams
@returns Stream transform.