add setting to disable welcome message

misc/toxic.conf.example

@@ -23,6 +23,9 @@ ui = {
   // true to show others when you're typing a message in 1-on-1 chats
   show_typing_self=true;
 
+  // true to show the welcome message on startup
+  show_welcome_msg=true;
+
   // maximum lines for chat window history
   history_size=700;
 };