Impl futures::stream::Stream for Controller/Listener

[ratmice]

In the example tokio usage in #45 it spawns an async task for each controller,
I thought it would be nice to be able to treat each controller as a stream and poll all controllers from a StreamMap, I implemented stream for Listener too, but didn't actually end up using it.

This allows something to the effect of the following to work, with the Controller automatically removed from the stream map on Event::Disconnect.

let mut stream_map = StreamMap::new();
loop {
    tokio::select! {
        c = &mut listener => {
            stdout.write_all(format!("Listener: {:?}\n", c).as_bytes()).await?;
            stream_map.insert(c.id(), c);     
        }
        Some(e) = stream_map.next() => stdout.write_all(format!("Event: {:?}", e).as_bytes()).await?,
    }
}

The main reason I added it in it's own feature is because it adds a new dependency on the futures crate.
Given that I wasn't sure if you would not want it to be in by default.

[AldaronLau]

@ratmice Thanks!

I think it makes sense behind a feature, so looks good to merge to me.