Support promises for callbacks like onDelete

[yaswanthmaddula-sureify]

I would like to have confirmation modal, if user tries to delete certain important keys in the json. As the onDelete callback is synchronous as of now, the only way for me to achieve this is using window.confirm(). It would be useful if there is a way like promises to support such usecases.

[CarlosNZ]

Thanks for the suggestion, I've started an issue to do this: #117