Add childrenClassName props and Firing children as dragged if parent dragged

[ihfazhillah]

Firstly, thank you for this awesome project, even it flagged as not maintained.

secondly, i made some changes from your codebase caused by our needing behaviour.

this change, i make a additional props called childrenClassName that make styling easier, and we can control children style even we use media css query.

we can pass it into Nestable component like childrenStyle, renderItem, etc.

also, i make a change that make children props called isDragging also true when we drag its parent.