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/weibo.py

@@ -28,6 +28,8 @@ class WeiboExtractor(Extractor):
     def __init__(self, match):
         Extractor.__init__(self, match)
         self._prefix, self.user = match.groups()
+
+    def _init(self):
         self.retweets = self.config("retweets", True)
         self.videos = self.config("videos", True)
         self.livephoto = self.config("livephoto", True)
@@ -228,6 +230,9 @@ class WeiboUserExtractor(WeiboExtractor):
         ("https://www.weibo.com/p/1003062314621010/home"),
     )
 
+    def initialize(self):
+        pass
+
     def items(self):
         base = "{}/u/{}?tabtype=".format(self.root, self._user_id())
         return self._dispatch_extractors((