code: function readDirsParallel(dirs, cb) {
    var funcs = [];
    dirs.forEach(function(dir) {
      funcs.push(readDirAbs.bind(null, dir));
    });
    async.parallel(funcs, function(err, results) {
      if (err) {
        return cb(err, null);
      }
      // Flatten results and return
      var flat = [].concat.apply([], results);
      cb(null, flat);
    }
    );
  }
docs: fs.readdir list of paths in parallel. used when `dir` arg is an array.