Feature Request: add support for checkboxes

[mviens]

Please consider adding support for checkboxes as the available options for a Material-UI checkbox tree are currently not very good. Thanks for your work so far.

[shallinta]

For now, we can do this with renderLabel function by return a Checkbox in label and keep its check-status in state data. I have tried this but found that it was not a good plan. Inplementing the click event callback would be the biggest problem.

So..
I will consider supporting this feature. (Maybe not very soon..