Reject subcommands that contain spaces in them

It's rather surprising that we can do this:

```
use argh::FromArgs;

#[derive(FromArgs)]
#[argh(description="")]
struct Cmd {
#[argh(subcommand)]
_sub: SubCmd,
}

#[derive(FromArgs)]
#[argh(subcommand, name = "sub sub", description="")]
struct SubCmd {}

fn main() {
let _cmd: Cmd = argh::from_env();
}
```

We can then execute this by running `cargo run -- "sub sub"`. We should probably explicitly reject this.