support for natural sorting for isotope

Sorting for isotope is in lexicographic ordering.
This can be odd. Elements ['1_file', '2_file', '20_file'] are sorted in this way:
1_file
20_file
2_file

With this pull request can be enable natural ordering, so:
1_file
2_file
20_file

Credits: https://github.com/overset/javascript-natural-sort/

Configure:
```javascript
$element.isotope({
    sortBy : sortName,
    sortAscending: true,
    naturalSorting : true
});
```