Added functions to create/destroy event_attrs to plugin interface.

Function names were chosen to keep consistency with the rest of the
API.

src/version-script

@@ -1,6 +1,8 @@
 EXPORTED {
 	global:
 		hexchat_hook_command;
+		hexchat_event_attrs_create;
+		hexchat_event_attrs_free;
 		hexchat_hook_server;
 		hexchat_hook_server_attrs;
 		hexchat_hook_print;
@@ -27,6 +29,7 @@ EXPORTED {
 		hexchat_plugingui_add;
 		hexchat_plugingui_remove;
 		hexchat_emit_print;
+		hexchat_emit_print_attrs;
 		hexchat_list_time;
 		hexchat_gettext;
 		hexchat_send_modes;