allow filtering '--list-extractors' results

with blacklist/whitelist syntax, e.g.

--list-extractors pixiv
--list-extractors pixiv:user pixiv:work
--list-extractors :search

gallery_dl/util.py

@@ -760,8 +760,9 @@ def build_extractor_filter(categories, negate=True, special=None):
     if catsub:
         def test(extr):
             for category, subcategory in catsub:
-                if category in (extr.category, extr.basecategory) and \
-                        subcategory == extr.subcategory:
+                if subcategory == extr.subcategory and (
+                        category == extr.category or
+                        category == extr.basecategory):
                     return not negate
             return negate
         tests.append(test)