decouple extractor initialization

Introduce an 'initialize()' function that does the actual init
(session, cookies, config options) and can called separately from
the constructor __init__().

This allows, for example, to adjust config access inside a Job
before most of it already happened when calling 'extractor.find()'.

gallery_dl/extractor/aryion.py

@@ -189,9 +189,11 @@ class AryionGalleryExtractor(AryionExtractor):
 
     def __init__(self, match):
         AryionExtractor.__init__(self, match)
-        self.recursive = self.config("recursive", True)
         self.offset = 0
 
+    def _init(self):
+        self.recursive = self.config("recursive", True)
+
     def skip(self, num):
         if self.recursive:
             return 0
@@ -217,9 +219,11 @@ class AryionTagExtractor(AryionExtractor):
         "count": ">= 5",
     })
 
-    def metadata(self):
+    def _init(self):
         self.params = text.parse_query(self.user)
         self.user = None
+
+    def metadata(self):
         return {"search_tags": self.params.get("tag")}
 
     def posts(self):