[common] add a 'kwdict' member to extractor instances

to allow setting general metadata at any point and without having to
rely on a manually implemented 'metadata()' method

gallery_dl/extractor/common.py

@@ -54,6 +54,7 @@ class Extractor():
         self.url = match.string
         self.match = match
         self.groups = match.groups()
+        self.kwdict = {}
 
         if self.category in CATEGORY_MAP:
             self.category = CATEGORY_MAP[self.category]