Use the Default trait for enums used as options

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 :

```rust
#[derive(argh::FromArgs)]
struct Args {
/// output format
#[argh(option)]
format: Option,
}

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

Using the Default trait :

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

[Case in my own code, just for context.](https://github.com/jRimbault/ddh/blob/cbcecff97202183bebaacefd5c6d60d51b0551d8/src/main.rs#L8-L37)