[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]

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: