Allow closure within column 'output' option

This change allows the column definition's 'output' option to be a closure.

In my particular use case, I had a column containing a JSON string that was automatically converted to an array in my model's get[Field]Attribute() call. This was causing 'array-to-string' conversion issues with Administrator, and I wanted to have better control on how that looked, anyway.

My Administrator model config now looks something like:
```php
return array(
    'title'       => 'Attributes',
    'single'      => 'Attribute',
    'model'       => 'Attribute',

    'columns'     => array(
        'code' => array(
            'title' => 'Code'
        ),
        'config'         => array(
            'title' => 'Config',
            'output' => function($value) {
                return join(', ', array_values($value));
            }
        )
    ),
   [...]
);
```