Rework logic for insert_url

Add "prepend" option to control where to add insert_url.

base/pref.ini

@@ -10,6 +10,8 @@ default_url=
 
 ;URLs to insert before subscription links, can be used to add node(s) to all exported subscriptions, use "|" to separate multiple subscription links, supports local files/URL
 insert_url=
+;Prepend inserted URLs to the node list. Nodes in insert_url will be added to groups first with non-group-specific match pattern.
+prepend_insert_url=true
 
 ;Exclude nodes which remarks match the following patterns. Supports regular expression.
 exclude_remarks=(到期|剩余流量|时间|官网|产品)