Adds the ability to include relations when using the UpdatedAt observer

Putting this up here to get thoughts on the feature/implementation. I thought this would be a useful feature as often an ORM entity consists of multiple models.

My example is that a blog post has many "i18ns", translatable data stored in a separate table (so another model), when any of the i18ns are updated I want to be able to reflect that change on the parent blog post. This observer will allow this to work.

```php
$blog_post->i18ns[$i18n_id]->title = 'My new title';
$blog_post->save();
// $blog_post->updated_at is now "now()"
```

Docs are on the way.