Replace the popup-content if it is already visible

Useful for form submits:

```
    $(document).on('click', '.popup', function (e) {
        e.preventDefault();
        var p = new Popelt({});
        $.get(e.target.href + '?mode=popup', function (data) {
            var content = $(data);
            $(document).on('submit', $('form', content), function (e) {
                e.preventDefault();
                $.post(e.target.action + '?mode=popup', $(e.target).serialize(), function (data) {
                    var submitted = $(data);
                    p.setContent(submitted);
                    p.show();
                });
            });
            p.setContent($(data));
            p.show();
        });
    })
```