Custom Add To Cart Messages

functions.php

<?php
/**
 * Custom Add To Cart Messages
 * Add this to your theme functions.php file
 **/
add_filter( 'woocommerce_add_to_cart_message', 'custom_add_to_cart_message' );
function custom_add_to_cart_message() {
	global $woocommerce;

	// Output success messages
	if (get_option('woocommerce_cart_redirect_after_add')=='yes') :

		$return_to 	= get_permalink(woocommerce_get_page_id('shop'));

		$message 	= sprintf('<a href="%s" class="button">%s</a> %s', $return_to, __('Continue Shopping &rarr;', 'woocommerce'), __('Product successfully added to your cart.', 'woocommerce') );

	else :

		$message 	= sprintf('<a href="%s" class="button">%s</a> %s', get_permalink(woocommerce_get_page_id('cart')), __('View Cart &rarr;', 'woocommerce'), __('Product successfully added to your cart.', 'woocommerce') );

	endif;

		return $message;
}
/* Custom Add To Cart Messages */
?>

How about changing the html output of the <a> tag? I need to include an id and a custom data- attribute. E.g id="open_cart" and data-cart="opened" - Manually setting these, just the id is returned.