[options] add '--compat' command-line option

docs/options.md

@@ -14,6 +14,7 @@
     --user-agent UA             User-Agent request header
     --clear-cache MODULE        Delete cached login sessions, cookies, etc. for
                                 MODULE (ALL to delete everything)
+    --compat                    Restore legacy 'category' names
 
 ## Update Options:
     -U, --update                Update to the latest version

gallery_dl/option.py

@@ -286,6 +286,11 @@ def build_parser():
         help="Delete cached login sessions, cookies, etc. for MODULE "
              "(ALL to delete everything)",
     )
+    general.add_argument(
+        "--compat",
+        dest="category-map", nargs=0, action=ConfigConstAction, const="compat",
+        help="Restore legacy 'category' names",
+    )
 
     update = parser.add_argument_group("Update Options")
     if util.EXECUTABLE: