Allow dropdown select event propagation

Selecting a dropdown item (via `click` or `touchstart`) cancels the event and emits a `select` event. As it doesn't bubble and the original event is stopped, we can't listen for it without attaching an event directly to the dropdown.

This pull request fixes the issue by allowing the event to bubble. The following becomes possible:

```javascript
on(document, '[data-toggle=dropdown]:select', function(e) {
    console.log(e.selectedItem);
});
```