Make italic text also editable

[mbehzad]

Hi,
The current selector for all elements that are recognized as editable (block-renderer.js#L81) lists the strong element but not em. I'm wondering if it is by design that if a table cell has only bold text, it should be editable in the side-kick library but with italic text not. Or the selector is simply missing? In that case let me know and I can open a PR.

cheers
Mehran