add Flag.flag to help text

[dastrobu]

Currently, the help text shows only the long flags, suche as e.g. --verbose, but not the short flag -f which can be configured via

let main = command(
    Flag("verbose", flag: "v")
) {
    // ...
}

[kylef]

This feature has been added to https://github.com/kylef/Commander/releases/tag/0.9.1