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/design.rst

@@ -171,6 +171,18 @@ Also see the :doc:`/becomingbig` section of the documentation for some
 inspiration for larger applications based on Flask.
 
 
+Async-await and ASGI support
+----------------------------
+
+Flask supports ``async`` coroutines for view functions, and certain
+others by executing the coroutine on a seperate thread instead of
+utilising an event loop on the main thread as an async first (ASGI)
+frameworks would. This is necessary for Flask to remain backwards
+compatibility with extensions and code built before ``async`` was
+introduced into Python. This compromise introduces a performance cost
+compared with the ASGI frameworks, due to the overhead of the threads.
+
+
 What Flask is, What Flask is Not
 --------------------------------