The callbacks onexit and oncomplete can return true or false for avoid the propagation.

Hi.

I need this feature, then I have written ;) , it is similtar to a events propagations in other javascript libraries, true = continue propagation and false = stop propagation.

And this is a example:
```
steps = [
              { 
                intro: "Hello <b>world!</b>"
              },
              {
                element: '#step1',
                intro: "This is a tooltip."
         }];

introJs()
         .setOption({steps: my_steps})
         .onexit(function() {
                  flag_propagation = confirm("Do you want to exit the tour?");
                  return flag_propagation;
         })
```

Regards.