Feature Request: Support for Subcommands with Clap Traits

[rrrodzilla]

I've noticed that the crate doesn't currently support structures like this:

enum Command {
    PrimaryCommand {
        subcommand: SubCommand,
    }
}
enum Subcommand {
    PrimaryCommand {
        some_arg: String,
    }
}

The issue arises because Subcommand doesn't implement necessary clap traits. It would be great if support could be added for this. Do you have an idea of how you'd do this? I don't mind assisting as well.

Nice work with this crate. 👍🏼