Adds ability to add custom classes when adding a new block via class property in data object

[alberttoledo]

Custom classes for blocks

Just like passing a src to an image block, I would love to be able to add a custom class to a block when creating it.

Drawback is to remove the class, we would need to replace it with a new block without that class.

I am using the library for the first time today, and this occurred to me as I needed it myself, but it might be a premature request since I am not very familiar. I thought I would open a PR instead of an issue but if there is a better way to do this, or a good reason not to do it, please let me know.

Thanks for the library!