Use the Default trait for enums used as options

[jRimbault]

It should be possible to use an enum as the possible value of an option, without wrapping it in an Option when and if the enum implements Default.

Current example :

#[derive(argh::FromArgs)]
struct Args {
    /// output format
    #[argh(option)]
    format: Option<OutputFormat>,
}

let args: Args = argh::from_env();
let format = args.format.unwrap_or_default();

Using the Default trait :

#[derive(argh::FromArgs)]
struct Args {
    /// output format
    #[argh(option)]
    format: OutputFormat, // enum should implement Default
}

Case in my own code, just for context.

[benbrittain]

I would definitely accept a PR for this, I've thought this in the past as well

[jRimbault]

I've yet to learn how to write derive macros. I have time these days, maybe I'll give it shot.