Async support and more

[ssrlive]

Hi @Detegr ,

After some years of independent development, the CtrlC2 I maintain has fully met my expectations.

Now I hope to contribute back to the CtrlC crate.

The main improvements of Ctrlc2 to Ctrlc are,

1. The return value of the function set_handler is a thread handle, which can be used to wait for its end.

2. The parameter passed into the function set_handler is a closure, which returns a boolean value, which is used to represent whether the user is willing to end the program.

fn main() {
    let handle = ctrlc2::set_handler(move || {
        println!(" ");
        println!("Ctrl-C received, ready to exiting...");
        true
    })
    .unwrap();
    println!("Waiting for Ctrl-C...");
    handle.join().unwrap();
    println!("Got it! Exiting...");
}

3. The AsyncCtrlC structure is implemented, which is a Future that users can use for any asynchronous runtime such as tokio or async-std.

async fn async_main() {
    let ctrlc = ctrlc2::AsyncCtrlC::new(move || {
        println!("Ctrl-C received!");
        true
    })
    .expect("cannot create Ctrl+C handler");
    println!("Waiting for Ctrl-C...");
    ctrlc.await.unwrap();
    println!("Got it! Exiting...");
}

If you think Ctrlc2 is a good improvement, I'd be happy to submit a PR to merge it back into CtrlC.