[path] implement dynamic length directories (#1350)

append directory segments for each item of a list (or general non-string
iterable), which can be returned with the 'I' specifier

gallery_dl/extractor/bbc.py

@@ -18,8 +18,7 @@ class BbcGalleryExtractor(GalleryExtractor):
     """Extractor for a programme gallery on bbc.co.uk"""
     category = "bbc"
     root = "https://www.bbc.co.uk"
-    directory_fmt = ("{category}", "{path[0]}", "{path[1]}", "{path[2]}",
-                     "{path[3:]:J - /}")
+    directory_fmt = ("{category}", "{path:I}")
     filename_fmt = "{num:>02}.{extension}"
     archive_fmt = "{programme}_{num}"
     pattern = rf"{BASE_PATTERN}[^/?#]+(?!/galleries)(?:/[^/?#]+)?)$"