docs: function run (files, callback) {
    let self = this,
      i = 0;
  
    function next(err, files) {
      let mw = self.middleware[i++];
  
      if (mw) {
        mw(files, self, next);
      }
      else {
        callback(null, files);
      }
  
    };
  
    next(null, files);
  }