Feature Request: choices

Similar to the [click](https://click.palletsprojects.com/en/8.1.x/options/#choice-options) arg parse library from Python, it would be nice if `argh` had a similar feature. I am new to rust, so I can't suggest a good interface, but perhaps something like:

```rust
#[derive(FromArgs, Debug)]
struct Args {
#[argh(choice, choices = vec!["foo", "bar", "baz"])]
thing_type: String
}
```

but an enum might be a better option:

```rust
#[derive(Debug)]
enum Choice {
Foo, Bar, Baz
}

#[derive(FromArgs, Debug)]
struct Args {
#[argh(choice, choices = Choice)]
thing_type: Choice
}
```

Perhaps this is possible already, but I couldn't figure out how to do so given the example and readme. Happy to close this if it is already possible.

Thanks!