Model Virtual Fields

You should be able to apply virtual fields to models. These can either be closures, or scalar values, or a combination.

Will look something like this.

```php
<?php
$model->virtual('scalar', 'blah');
$model->virtual('closure', function($field) {
    // $model === $this
    
    return 'blah';
});
$model->virtual('combination', [
    'scalar' => 'blah',
    'closure' => function($field) {
        return 'blah';
    }
]);
```