Added ability to skip items with disabledClass. Useful when sorting over disabled or hidden items.

[poscar]

Items with the class specified by the disabledClass option will be skipped when sorting another item over them.

This is useful if you have a nested list where some items can be disabled/hidden but you still need to be able to properly sort over them. Without this feature sorting over hidden items behaves erratically when changing nesting levels.

Thanks!