added --autonumber-start NUMBER as a command line option to be able to offset the index at which autonumber formats filenames

youtube_dl/__init__.py

@@ -321,6 +321,7 @@ def _real_main(argv=None):
         'listformats': opts.listformats,
         'outtmpl': outtmpl,
         'autonumber_size': opts.autonumber_size,
+        'autonumber_start': opts.autonumber_start,
         'restrictfilenames': opts.restrictfilenames,
         'ignoreerrors': opts.ignoreerrors,
         'force_generic_extractor': opts.force_generic_extractor,