/**
 * Add post categories in class
 *
 * @param  mixed $output return html.
 * @return $output
 */
function reign_post_categories_callback( $output ) {
	global $post;
	$post_id       = $post->ID;
	$post_category = get_the_category( $post->ID );
	$cat_name      = '';
	foreach ( $post_category as $post_key => $post_val ) {
			$cat_name .= $post_val->slug . ' ';
	}
	$output = str_replace( 'cat-links', 'cat-links ' . $cat_name, $output );
	return $output;
}
add_filter( 'reign_post_categories', 'reign_post_categories_callback' );