Add freeze prop to stop dragging events

[alfonsogarciacaro]

I couldn't prevent the tree from being modified in the onChange event so this PR just adds a new prop to make it easy to stop dragging events. I didn't name the prop editable in order not to break compatibility with current code.

If you think this is a good idea, please feel free to change the prop name or modify the code in any other way.

Thanks a lot for this useful React component!

[DevAlien]

Could you try to solve the conflicts?

[alfonsogarciacaro]

If someone is planning to actually merge this PR, yes ;)

[benjamindean]

@wangzuo Any plans on merging this?