Change WebSocket API to use an extractor (#121)

Fixes https://github.com/tokio-rs/axum/issues/111

Example usage:

```rust
use axum::{
    prelude::*,
    extract::ws::{WebSocketUpgrade, WebSocket},
    response::IntoResponse,
};

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

async fn handler(ws: WebSocketUpgrade) -> impl IntoResponse {
    ws.on_upgrade(handle_socket)
}

async fn handle_socket(mut socket: WebSocket) {
    while let Some(msg) = socket.recv().await {
        let msg = if let Ok(msg) = msg {
            msg
        } else {
            // client disconnected
            return;
        };

        if socket.send(msg).await.is_err() {
            // client disconnected
            return;
        }
    }
}
```

src/extract/mod.rs

@@ -254,6 +254,10 @@ pub mod connect_info;
 pub mod extractor_middleware;
 pub mod rejection;
 
+#[cfg(feature = "ws")]
+#[cfg_attr(docsrs, doc(cfg(feature = "ws")))]
+pub mod ws;
+
 mod content_length_limit;
 mod extension;
 mod form;
@@ -292,6 +296,11 @@ pub mod multipart;
 #[doc(inline)]
 pub use self::multipart::Multipart;
 
+#[cfg(feature = "ws")]
+#[cfg_attr(docsrs, doc(cfg(feature = "ws")))]
+#[doc(inline)]
+pub use self::ws::WebSocketUpgrade;
+
 #[cfg(feature = "headers")]
 #[cfg_attr(docsrs, doc(cfg(feature = "headers")))]
 mod typed_header;