Added the Accordion open switch passthrough.

[zinigor]

This PR makes it possible for the Accordion to pass through the 'isOpen' property down to the collapsible section. This way the fields can use the following setup to remain open by default:

    $fields->render_field('accordion-open-by-default', [
        'label'   => 'Accordion',
        'type'    => 'accordion',
        'title'   => 'Simple Open Accordion',
        'isOpen'  => true,
        'value'   => $fields->fetch_value('accordion'),
        'fields'  => [
            [
                'label' => 'Text',
                'type'  => 'text',
                'name'  => 'text_name',
            ],
        ]
    ]);

[nicolas-jaussaud]

Thanks for the PR and for updating the examples/tests as well!

I left a small comment regarding the field registration in the example, but other than that feel free to merge!