core: improve the string_replace_regex function (add reference char, change syntax for match refs)

The reference char is now an argument for the function.
The references are now $0 .. $99 and $+ was added (last match, with
highest number).
The syntax to replace a match with one char is now: $.cN or $.c+
(for example: "$.*3").

src/plugins/trigger/trigger-callback.c

@@ -114,7 +114,8 @@ trigger_callback_replace_regex (struct t_trigger *trigger,
 
         value = weechat_string_replace_regex (ptr_value,
                                               trigger->regex[i].regex,
-                                              trigger->regex[i].replace_eval);
+                                              trigger->regex[i].replace_eval,
+                                              '$');
         if (!value)
             continue;