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.

setup.py

@@ -9,5 +9,8 @@ setup(
         "itsdangerous>=0.24",
         "click>=5.1",
     ],
-    extras_require={"dotenv": ["python-dotenv"]},
+    extras_require={
+        "async": ["asgiref>=3.2"],
+        "dotenv": ["python-dotenv"],
+    },
 )