Add Authorization header filters

I took a shot at adding support for Authorization header filters. I would like to know whether you like the approach I took: https://github.com/algermissen/warp/blob/add-auth-headers/src/filters/authorization.rs

If so, I'll work on making the code better and submit a PR after that - given I am relatively new to Rust, there is a lot of stuff that needs improvement (eg reduce String usage), so please do not judge that yet.

Some questions:

• I think an addition to Rejections is needed to support the 401 challenge response, yes?
• Since the 401 response includes parameters that must be settable by the code using warp, Rejections must somehow support additional information that can be set (eg the Basic auth filter would need to return a rejection with realm information):

    let text3 = warp::path("protected").
        and(warp::authorization::basic("myRealm"))
        .map(|b: warp::authorization::BasicCredentials| {
            format!("User: {} Passwd: {}", b.user,b.password)
        });

• Or would you rather return an error response instead of a rejection since from the point of view of the HTTP authentication protocol, the thing to do is to return 401 + challenge. Allowing for further rejection handling does not really make sense.