[Request] better raw identifier support

```rust
#[derive(FromArgs, Debug)]
/// tool
pub struct Config {
#[argh(option)]
/// run forever. Exit with Ctrl-C.
pub r#loop: bool,
}
```

```
$ tool --loop
Required options not provided:
--r-loop
```

Obviously, here the flag should be `loop` not `r-loop` :)