Add async support

This allows for async functions to be passed to the Flask class
instance, for example as a view function,

    @app.route("/")
    async def index():
        return "Async hello"

this comes with a cost though of poorer performance than using the
sync equivalent.

asgiref is the standard way to run async code within a sync context,
and is used in Django making it a safe and sane choice for this.

docs/index.rst

@@ -59,6 +59,7 @@ instructions for web development with Flask.
    patterns/index
    deploying/index
    becomingbig
+   async_await
 
 
 API Reference