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/async_await.rst

@@ -0,0 +1,46 @@
+.. _async_await:
+
+Using async and await
+=====================
+
+.. versionadded:: 2.0
+
+Routes, error handlers, before request, after request, and teardown
+functions can all be coroutine functions if Flask is installed with
+the ``async`` extra (``pip install flask[async]``). This allows code
+such as,
+
+.. code-block:: python
+
+    @app.route("/")
+    async def index():
+        return await ...
+
+including the usage of any asyncio based libraries.
+
+
+When to use Quart instead
+-------------------------
+
+Flask's ``async/await`` support is less performant than async first
+frameworks due to the way it is implemented. Therefore if you have a
+mainly async codebase it would make sense to consider `Quart
+<https://gitlab.com/pgjones/quart>`_. Quart is a reimplementation of
+the Flask using ``async/await`` based on the ASGI standard (Flask is
+based on the WSGI standard).
+
+
+Decorators
+----------
+
+Decorators designed for Flask, such as those in Flask extensions are
+unlikely to work. This is because the decorator will not await the
+coroutine function nor will they themselves be awaitable.
+
+
+Other event loops
+-----------------
+
+At the moment Flask only supports asyncio - the
+:meth:`flask.Flask.ensure_sync` should be overridden to support
+alternative event loops.