Setting: only write in console upon muc mention

`/console muc mention` additionally to `first|none|all`.

Fix https://github.com/profanity-im/profanity/issues/1371

src/command/cmd_ac.c

@@ -913,6 +913,7 @@ cmd_ac_init(void)
     console_msg_ac = autocomplete_new();
     autocomplete_add(console_msg_ac, "all");
     autocomplete_add(console_msg_ac, "first");
+    autocomplete_add(console_msg_ac, "mention");
     autocomplete_add(console_msg_ac, "none");
 
     autoping_ac = autocomplete_new();