Allow dot notation for customer attributes

I'd like to propose allowing the use of dot notation for accessing nested attributes via relationships when using custom attributes.

Eg: For a table of posts, instead of adding an accessor attribute to the Post model `getAuthorFullNameAttribute()`, using dot notation, you can simply access the authors full_name attribute directly, through the relationship:

```
Column::make(__('Author'), 'author.full_name')
                ->customAttribute()
                ->searchable()
                ->sortable(),
```

Thanks for this awesome package. Truly is a pleasure.

Author: izzyp

resources/views/includes/_body.blade.php

@@ -53,7 +53,7 @@ class="{{ $this->setTableDataClass($column->attribute, Arr::get($model->toArray(
                                 @endif
                             @else
                                 @if ($column->isCustomAttribute())
-                                    {{ $model->{$column->attribute} }}
+                                    {{ data_get($model, $column->attribute) }}
                                 @else
                                     {{ Arr::get($model->toArray(), $column->attribute) }}
                                 @endif