code: function(obj, keys) {
      var newObj = {};
      for (var i = 0, len = keys.length; i < len; i++) {
        if (keys[i] in obj) {
          newObj[keys[i]] = obj[keys[i]];
        }
      }
      return newObj;
    }
docs: Makes a shallow copy of `obj` (like `_extend`) but filters its properties based on a list of `keys` to keep.
  The inverse of `_omit`, mostly. The big difference is that these properties do NOT need to be enumerable to
  be kept.
  
  @returns A new filtered object.
  @private