Add an onElementNotFound() event

[Nerpson]

Hello,

Such event would be really useful, especially in my case when I'm using a JS page scroller. This would help me to know that the page scroller isn't fully loaded yet.

new Tour({
   [...],
    onElementNotFound: function() {
        scroller.doSmth();
    },
    [...]
});

Regards

[IGreatlyDislikeJavascript]

onElementUnavailable: option added to my fork

https://github.com/IGreatlyDislikeJavascript/bootstrap-tour https://github.com/IGreatlyDislikeJavascript/bootstrap-tour/blob/master/build/js/bootstrap-tour.js