Zero-pad numbers in Selection menu for indentation purposes

[stdedos]

I believe feature is self-explainatory. If you have 10 items, then numbering would go 01 etc

I could also attempt to provide a PR, but I haven't been in that culture, so, I am not outright attempting it.

I believe that this is the point of change: https://github.com/pmbarrett314/curses-menu/blob/c76fc00ab9d518eab275e55434fc2941f49c6b30/cursesmenu/selection_menu.py#L15

with some notepad (like pad_no = log10(len(strings)) / index.zfill(pad_no))