Make it easier to set up HTTP/2 with an HTTP/1 fallback.

[thosakwe]

Currently, you need both an instance of AngelHttp2 and AngelHttp:

var http = AngelHttp(app);
var http2 = AngelHttp2(app, ...);
http2.onHttp1.listen(http.handleRequest);
await http2.startServer();

It should be easier to do this, perhaps behind an allowHttp1 flag:

var http2 = AngelHttp2(app, allowHttp1: true);
await http2.startServer();

Open to suggestions/comments!