Add keybinding for setting null

For some observables, `null` is a valid state. This change registers two key combinations for pushing null back to the bound observable.

Two things to consider:
* I'm using both the DELETE key and CTRL+BACKSPACE, but having second thoughts about the latter, because ...
* I only changed the behavior for the current cell (via `onKeyDown`), and realize that it might be useful to reserve CTRL+DELETE (or CTRL+BACKSPACE) for setting the _selection range_ to null (similar to CTRL+ENTER). 

The only reason for keeping CTRL+BACKSPACE is because the Mac keyboard doesn't have a DELETE key (and it appears that FN+BACKSPACE *works* like DELETE, but doesn't send the same keycode)

Also bumped patch version.