Restore "find_under_expand" native functionality

Being able to add a selection to a multiple selection set is, to me, one of the killer feature of sublime text. So I arranged the keyboard map to bind ctrl-alt-d to "find_under_expand" behavior.

It's the most natural keymap to me, but might not be for anyone else. I think this feature should be binded to a keyboard shortcut in any way, even though it's not a native emacs feature.

Let me know what you think.