Add support for websockets (#3)

Basically a copy/paste of whats in warp.

Example usage:

```rust
use tower_web::{prelude::*, ws::{ws, WebSocket}};

let app = route("/ws", ws(handle_socket));

async fn handle_socket(mut socket: WebSocket) {
    while let Some(msg) = socket.recv().await {
        let msg = msg.unwrap();
        socket.send(msg).await.unwrap();
    }
}
```

examples/websocket/script.js

@@ -0,0 +1,9 @@
+const socket = new WebSocket('ws://localhost:3000/ws');
+
+socket.addEventListener('open', function (event) {
+    socket.send('Hello Server!');
+});
+
+socket.addEventListener('message', function (event) {
+    console.log('Message from server ', event.data);
+});