code: function transform() {
        return through.obj(function(file, enc, cb) {
          return config.transform(file, enc, cb, opts);
        }, function(cb) {
          if (typeof config.onFinish === 'function') config.onFinish(opts, cb);
          else cb();
        });
      }
docs: Function to transform files passed to the plugin. If the plugin is being called in a stream, this function is returned as-is. If the plugin is being called standalone, an ad-hoc stream is created and the input files are piped to this function.
  If a plugin defines an `onFinish` callback (through `opts.onFinish`), it will be run when all files have been processed.
  @returns {stream.Transform} Transform stream function that modifies files.