[Feature] Add options / hooks for subscribing to Bootstrap modal events

[tiesont]

See #733 for the genesis of this feature.

We'll be adding the ability to subscribe to the events raised by Bootstrap's modal function:

• onShow
  Handles the show.bs.modal event.
• onShown
  Handles the shown.bs.modal event.
• onHide
  Handles the hide.bs.modal event.
• onHidden
  Handles the hidden.bs.modal event.

Once implemented, this:

let dialog = bootbox.alert({
    message: 'I have a custom shown handler!'
});

dialog.on('shown.bs.modal', function(e) {
    /* code to execute when shown.bs.modal is raised */
})

becomes this:

bootbox.alert({
    message: 'I have a custom shown handler!',
    onShown: function(e) {
        /* code to execute when shown.bs.modal is raised */
    }
});