Add webhook support.

Allows for usage of either `requests` and `aiohttp` when used in
"Standalone" mode.

Fixes #704

docs/conf.py

@@ -50,6 +50,7 @@ extlinks = {
 
 rst_prolog = """
 .. |coro| replace:: This function is a |corourl|_.
+.. |maybecoro| replace:: This function *could be a* |corourl|_.
 .. |corourl| replace:: *coroutine*
 .. _corourl: https://docs.python.org/3/library/asyncio-task.html#coroutine
 """