add structured IRC event hooks

docs/plugin-api-additions/events.md

@@ -0,0 +1,26 @@
+# Semantic IRC hooks
+
+`on_join`, `on_part`, `on_quit`, `on_nick`, `on_kick`, `on_message`, and
+`on_notice` accept `(callback, userdata=None, priority=PRI_NORM)`. The general
+form is `hook_event(command, callback, userdata=None, priority=PRI_NORM)`.
+Callbacks receive `(event, userdata)`, where the immutable IRCEvent has
+command, prefix, nick, user, host, params, context, connection_id and time.
+`params` preserves the trailing parameter, including spaces; JOIN parameters
+also retain extended-join account/realname data when supplied by the server.
+
+These wrappers use existing server hooks, before core command processing;
+return the ordinary EAT_* constants. EAT_ZOITECHAT suppresses the entire IRC
+command (including client state updates), not just its text. Use EAT_NONE for
+observation. Other plugins' priority and eating rules remain unchanged.
+The context is the core-selected context, which can be the server tab when a
+query/channel does not yet exist. QUIT is one event per incoming command,
+not one callback per affected channel. `event.time` is the existing server-time
+attribute (zero when unavailable); other message tags are not exposed here.
+
+```python
+def joined(event, userdata):
+    event.context.prnt('{} joined {}'.format(event.nick, event.params[0]))
+    return zoitechat.EAT_NONE
+
+zoitechat.on_join(joined)
+```