(feat) Add mousewheel support

[derdritte]

To let mouse-wheel events change the current picture to the previous or next picture:

baguetteBox.run('.gallery', {
    'handleWheel': true
});

Default is false, that seemed appropriate to me.

The current implementation prevents default and, to avoid having to restructure when (user)options are loaded, the event-handler is always added to the overlay and the event-handler will check the option on each event. Naturally this is not ideal, but didn't think adding this small feature warranted that level of refactoring. I am not a JS-dev, any input is appreciated.

An (additional) option to allow inverting of the directions the scroll-events get translated to can be added easily, if desired.
I also read the contribution guidelines, but opening a ticket for such a minimal change seemed overkill, if the feature isn't welcome or relevant, the PR can be closed. If you want the discussion anyway, I'd be happy to open an issue.