Single dash is parsed as unrecognized argument

According to the Fuchsia command line rubric, and general Unix practice, a single dash is not allowed as a key or switch, but that's how argh treats it, because it doesn't have a special case for it like it does for `--`.

https://github.com/google/argh/blob/3f3c29726a21c4b541bb2b9aa2c592461897ded0/argh/src/lib.rs#L893

This means that to pass a single dash as a positional argument, you have to pass `-- -`, which is very unintuitive.