Add scrollbar support

The attached patch adds scrollbar support into the module. Default settings match to API defaults. Empty checking is in place to avoid the necessity of an update hook to backfill existing swiper template config. I think the addition of an update hook here would be an improvement, but this is a start.

Thank you so much for a great work on this one, as well as mousewheel ✨ Mousewheel support Needs review . I've tested both on 2.0.x version and just pushed it.

Version 1.0.x got some (important like) changes meanwhile so I assume patch won't work but we might consider re-rolling.

Automatically closed - issue fixed for 2 weeks with no activity.