Key and Param default value

[yonaskolb]

It would be awesome to have an optional default value for Param and Key. This would allow:

• setting the default value in the same place as the rest of the option configuration, instead of down in execute with option ?? defaultValue
• allow Param and Key to have non optional types (Param already supports non optional types but currently fails if value not supplied)
• allow an option to HelpMessageGenerator that would print the default value of options, perhaps after the description eg: " Defaults to \(defaultValue)"

@Key("--type", "-t", description: "The type of dump to output", defaultValue: .yaml)
private var dumpType: DumpType

[jakeheis]

This was near impossible when property wrappers were new and the Swift compiler crashed while doing just about anything with property wrappers. I think it's worth attempting this feature again now in Swift 5.4 though -- will implement when I get a chance