allow running a BaseExtractor for any URL

by prefixing it with '<base-category>:'

For example:
  shopify:https://partakefoods.com/products/crunchy-cookie-variety-pack
  gelbooru_v01:https://5naf.booru.org/index.php?page=post&s=view&id=46963

Available base categories are:
  mastodon, shopify, moebooru, gelbooru_v01, gelbooru_v02,
  reactor, foolslide, foolfuuka,  philomena

gallery_dl/extractor/common.py

@@ -571,7 +571,11 @@ class BaseExtractor(Extractor):
         if not self.category:
             for index, group in enumerate(match.groups()):
                 if group is not None:
-                    self.category, self.root = self.instances[index]
+                    if index:
+                        self.category, self.root = self.instances[index-1]
+                    else:
+                        self.root = group
+                        self.category = group.partition("://")[2]
                     break
         Extractor.__init__(self, match)
 
@@ -594,7 +598,10 @@ class BaseExtractor(Extractor):
                 pattern = re.escape(root[root.index(":") + 3:])
             pattern_list.append(pattern + "()")
 
-        return r"(?:https?://)?(?:" + "|".join(pattern_list) + r")"
+        return (
+            r"(?:" + cls.basecategory + r":(https?://[^/?#]+)|"
+            r"(?:https?://)?(?:" + "|".join(pattern_list) + r"))"
+        )
 
 
 class HTTPSAdapter(HTTPAdapter):