consistent cookie-related names

- rename every cookie variable or method to 'cookies_*'
- simplify '.session.cookies' to just '.cookies'
- more consistent 'login()' structure

gallery_dl/extractor/erome.py

@@ -65,7 +65,7 @@ class EromeExtractor(Extractor):
     def request(self, url, **kwargs):
         if self.__cookies:
             self.__cookies = False
-            self.session.cookies.update(_cookie_cache())
+            self.cookies.update(_cookie_cache())
 
         for _ in range(5):
             response = Extractor.request(self, url, **kwargs)