pyln: add ability to subscribe to all notifications.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/plugins/all_notifications.py

@@ -0,0 +1,17 @@
+#!/usr/bin/env python3
+from pyln.client import Plugin
+import sys
+
+
+plugin = Plugin()
+
+
+@plugin.subscribe("*")
+def on_any_notification(request, **kwargs):
+    plugin.log("notification {}: {}".format(request.method, kwargs))
+    if request.method == 'shutdown':
+        # A plugin which subscribes to shutdown is expected to exit itself.
+        sys.exit(0)
+
+
+plugin.run()