Add extractor for remote connection info (#55)

Fixes https://github.com/tokio-rs/axum/issues/43

With this you can get the remote address like so:

```rust
use axum::{prelude::*, extract::ConnectInfo};
use std::net::SocketAddr;

let app = route("/", get(handler));

async fn handler(ConnectInfo(addr): ConnectInfo<SocketAddr>) -> String {
    format!("Hello {}", addr)
}

// Starting the app with `into_make_service_with_connect_info` is required
// for `ConnectInfo` to work.
let make_svc = app.into_make_service_with_connect_info::<SocketAddr, _>();

hyper::Server::bind(&"0.0.0.0:3000".parse().unwrap())
    .serve(make_svc)
    .await
    .expect("server failed");
```

This API is fully generic and supports whatever transport layer you're using with Hyper. I've updated the unix domain socket example to extract `peer_creds` and `peer_addr`.

src/extract/mod.rs

@@ -260,12 +260,16 @@ use std::{
     task::{Context, Poll},
 };
 
+pub mod connect_info;
 pub mod extractor_middleware;
 pub mod rejection;
 
 #[doc(inline)]
 pub use self::extractor_middleware::extractor_middleware;
 
+#[doc(inline)]
+pub use self::connect_info::ConnectInfo;
+
 #[cfg(feature = "multipart")]
 #[cfg_attr(docsrs, doc(cfg(feature = "multipart")))]
 pub mod multipart;
@@ -904,6 +908,8 @@ where
 /// # hyper::Server::bind(&"".parse().unwrap()).serve(app.into_make_service()).await.unwrap();
 /// # };
 /// ```
+///
+/// [`Stream`]: https://docs.rs/futures/latest/futures/stream/trait.Stream.html
 #[derive(Debug)]
 pub struct BodyStream<B = crate::body::Body>(B);