Server-Sent Events (#75)

Example usage:

```rust
use axum::{prelude::*, sse::{sse, Event, KeepAlive}};
use tokio_stream::StreamExt as _;
use futures::stream::{self, Stream};
use std::{
    time::Duration,
    convert::Infallible,
};

let app = route("/sse", sse(make_stream).keep_alive(KeepAlive::default()));

async fn make_stream(
    // you can also put extractors here
) -> Result<impl Stream<Item = Result<Event, Infallible>>, Infallible> {
    // A `Stream` that repeats an event every second
    let stream = stream::repeat_with(|| Event::default().data("hi!"))
        .map(Ok)
        .throttle(Duration::from_secs(1));

    Ok(stream)
}
```

Implementation is based on [warp's](https://github.com/seanmonstar/warp/blob/master/src/filters/sse.rs)

examples/sse.rs

@@ -0,0 +1,50 @@
+use axum::{extract::TypedHeader, prelude::*, routing::nest, service::ServiceExt, sse::Event};
+use futures::stream::{self, Stream};
+use http::StatusCode;
+use std::{convert::Infallible, net::SocketAddr, time::Duration};
+use tokio_stream::StreamExt as _;
+use tower_http::{services::ServeDir, trace::TraceLayer};
+
+#[tokio::main]
+async fn main() {
+    tracing_subscriber::fmt::init();
+
+    // build our application with a route
+    let app = nest(
+        "/",
+        axum::service::get(
+            ServeDir::new("examples/sse")
+                .append_index_html_on_directories(true)
+                .handle_error(|error: std::io::Error| {
+                    Ok::<_, std::convert::Infallible>((
+                        StatusCode::INTERNAL_SERVER_ERROR,
+                        format!("Unhandled interal error: {}", error),
+                    ))
+                }),
+        ),
+    )
+    .route("/sse", axum::sse::sse(make_stream))
+    .layer(TraceLayer::new_for_http());
+
+    // run it
+    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
+    tracing::debug!("listening on {}", addr);
+    hyper::Server::bind(&addr)
+        .serve(app.into_make_service())
+        .await
+        .unwrap();
+}
+
+async fn make_stream(
+    // sse handlers can also use extractors
+    TypedHeader(user_agent): TypedHeader<headers::UserAgent>,
+) -> Result<impl Stream<Item = Result<Event, Infallible>>, Infallible> {
+    println!("`{}` connected", user_agent.as_str());
+
+    // A `Stream` that repeats an event every second
+    let stream = stream::repeat_with(|| Event::default().data("hi!"))
+        .map(Ok)
+        .throttle(Duration::from_secs(1));
+
+    Ok(stream)
+}

examples/sse/index.html

@@ -0,0 +1 @@
+<script src='script.js'></script>

examples/sse/script.js

@@ -0,0 +1,5 @@
+var eventSource = new EventSource('sse');
+
+eventSource.onmessage = function(event) {
+    console.log('Message from server ', event.data);
+}