Added panel support

As suggested in https://github.com/devinsays/customizer-library/issues/21

Usages:

```
    $section = 'example-section';

    $sections[] = array(
        'id' => $section,
        'title' => __( 'Example Section', 'demo' ),
        'priority' => '10',
        'description' => __( 'A description for the example section.', 'demo' ),
        'panel' => 'theme_options',
    );
```