Allow to use numbers as shortcuts, overriding counts

Allow to use numbers as shortcuts, overriding counts

... properly handling the special-case for `0`:

- `0J` is _not_ a count, because a count of zero makes no sense. It first
  invokes the `0` command, and then the `J` command.
- `20J` _is_ a count, repeating twenty times. In other words, pressing `2` and
  then `0` does _not_ run the `0` command (scroll to the far left) with a count
  of two. Instead, it sets the waiting count to twenty.

Fixes #587.