Flash messages mechanism, closes #790

tests/plugins/messages_output_renderer.py

@@ -0,0 +1,21 @@
+from datasette import hookimpl
+
+
+def render_message_debug(datasette, request):
+    if request.args.get("add_msg"):
+        msg_type = request.args.get("type", "INFO")
+        datasette.add_message(
+            request, request.args["add_msg"], getattr(datasette, msg_type)
+        )
+    return {"body": "Hello from message debug"}
+
+
+@hookimpl
+def register_output_renderer(datasette):
+    return [
+        {
+            "extension": "message",
+            "render": render_message_debug,
+            "can_render": lambda: False,
+        }
+    ]