Add append function

Hi

Love the library.

How do you feel about adding the append method that is currently missing.

```php
/**
* Append attributes to query when building a query.
*
* @param array|string $attributes
* @return $this
*/
public function append($attributes)
{
$this->appends = array_unique(
array_merge($this->appends, is_string($attributes) ? func_get_args() : $attributes)
);

return $this;
}
```

This way we can append dynamically and they'll be present when the final model is requested via toArray();

```php
$model->append('thing');

$model->toArray(); // includes ["thing" => ....]
```