Return $this in add_* methods to enable chaining

```php
$settings_page
	->add_section( array(
		'slug'          => 'general',
		'title'         => 'General',
		'settings_page' => 'totally_rad_theme_settings',
	) )->add_section( array(
		'slug'          => 'more',
		'title'         => 'More',
		'settings_page' => 'totally_rad_theme_settings',
	) );

$settings_page->get_section( 'general' )
	->add_field( array(
		'slug'  => 'footer_text',
		'title' => __( 'Footer Text', 'text-domain' ),
	) )
	->add_field( array(
		'slug'  => 'comment_note',
		'title' => __( 'Comment Note', 'text-domain' ),
	) );
```