[Propoal] Add multiple attribute support to Model::isDirty

This adds support for checking multiple attributes at once with Model::isDirty without breaking compatibility. 
Attributes to check can be passed as an array of attribute names or as multiple arguments:

```php
$model->isDirty(); // true if any attribute is dirty
$model->isDirty('foo'); // true if attribute 'foo' is dirty
$model->isDirty('foo', 'bar'); // true if 'foo' or 'bar' is dirty
$model->isDirty(['foo', 'bar', 'baz']); // true if at least one of 'foo', 'bar', or 'baz' is dirty
```