The CSS output module for Kirki.
TODO:
How the object will be instantiated:
// Instantiate the object for our config.
$my_output = Kirki_Output::get_instance( 'my_config' );
// Add a field.
new Kirki_Output_Field( 'config_id', $field );
// Generate the CSS after all fields have been added.
$my_output->generate_css();
// Get the final CSS.
$css = $my_output->get_css();