Get list of directories `dir` from asynchronously

@param {String} dir
@param {Function} cb
function getDirectories(dir, cb) {
  fs.readdir(dir, function(err, objects){
    if (err) { return cb && cb(err); }

    var dirs = objects.filter(function(item){
      return fs.statSync(path.join(dir, item)).isDirectory();
    });

    cb && cb(null, dirs)
  })
}