Combinatoric example

```no_run
#[derive(Debug, Clone)]
pub struct Options {
    number: u32,
}
pub fn options() -> OptionParser<Options> {
    let number = long("number").argument::<u32>("N").map(|x| x * 2);
    construct!(Options { number }).to_options()
}

fn main() {
    println!("{:?}", options().run())
}
```


Derive example

```no_run
fn twice_the_num(n: u32) -> u32 {
    n * 2
}

#[derive(Debug, Clone, Bpaf)]
#[bpaf(options)]
pub struct Options {
    #[bpaf(argument::<u32>("N"), map(twice_the_num))]
    number: u32,
}

fn main() {
    println!("{:?}", options().run())
}
```


Output

`map` don't make any changes to generated `--help` message


You can use `map` to apply arbitrary pure transformation to any input.
Here `--number` takes a numerical value and doubles it

$ app --number 10
Options { number: 20 }

But if function inside the parser fails - user will get the error back unless it's handled
in some way. In fact here execution never reaches `map` function -
[`argument`](NamedArg::argument) tries to parse `ten` as a number, fails and reports the error

$ app --number ten
Error: couldn't parse ten: invalid digit found in string

