Support options with only a short flag

[shadowspawn]

Commander originally expected there would always be both short and long flags for each option. Support got added for long-only (#18).

Things almost work if you do try defining only a short flag, but by accident, and the value for -a ends up getting stored for A.

.option(`-d`, `debug`)

(The simple work-around is to add a long flag, so this may not be much of an issue in practice as has not been raised in issues here. It did get mentioned as a minor annoyance in a good comparison article in 2018: https://pantas.net/node_argument_parsers/ )

[shadowspawn]

Released with Commander v6.0.0 : https://github.com/tj/commander.js/releases/tag/v6.0.0