Mark cloned slider items with class

[m41r]

Issue description:
It would be really helpful, if the cloned items has a marker class, e.g. tns-cloned, so we can exclude them easily with .tns-item:not(.tns-cloned)'.

Example:
With the library 'lightgallery.js', the items would appear twice in the thumbnails, because they are not excludable.

Tiny-slider version:
v2.9.2

Browser name && version:
Google Chrome 79.0.3945.130

OS name && version:
Windows 10 Pro