[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/job.py

@@ -228,6 +228,8 @@ class Job():
         kwdict["subcategory"] = extr.subcategory
         if self.metadata_http:
             kwdict.pop(self.metadata_http, None)
+        if extr.kwdict:
+            kwdict.update(extr.kwdict)
         if self.kwdict:
             kwdict.update(self.kwdict)
         if self.kwdict_eval: