Add-close-button method

@param {object} el - popup instance DOM node
function (el) {
            var genericCloseButton;

            if ($(el).data('popupoptions').closebuttonmarkup) {
                genericCloseButton = $(options.closebuttonmarkup).addClass(el.id + '_close');
            } else {
                genericCloseButton = '<button class="popup_close ' + el.id + '_close" title="Close" aria-label="Close"><span aria-hidden="true">×</span></button>';
            }

            if ($(el).data('popup-initialized')){
                $(el).append(genericCloseButton);
            }

        }