Add an ability to users to add their own custom form types

Example:
```php
$app->extend('form.type.custom_types', function($customTypes) {
    $customTypes[] = new DummyFormType();
    return $customTypes;
});
...
$form = $app['form.factory']->createBuilder()
    ->add('dummy-field', 'dummy')
    ->getForm();
```