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/__init__.py

@@ -202,12 +202,18 @@ def main():
             extractor.modules.append("")
             sys.stdout.write("\n".join(extractor.modules))
 
-        elif args.list_extractors:
+        elif args.list_extractors is not None:
             write = sys.stdout.write
             fmt = ("{}{}\nCategory: {} - Subcategory: {}"
                    "\nExample : {}\n\n").format
 
-            for extr in extractor.extractors():
+            extractors = extractor.extractors()
+            if args.list_extractors:
+                fltr = util.build_extractor_filter(
+                    args.list_extractors, negate=False)
+                extractors = filter(fltr, extractors)
+
+            for extr in extractors:
                 write(fmt(
                     extr.__name__,
                     "\n" + extr.__doc__ if extr.__doc__ else "",