Allow using different types of elements in columns

This pull request is to allow the use of different types of elements when creating columns using Simple-Grid. It replaces the :last-of-type selectors with :last-child ones. This kind of markup is then possible :

```html
<article class="grid">
    <div class="col-10-12"><!-- Content --></div>
    <aside class="col-2-12"><!-- Content --></div>
</article>
```