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

youtube_dl/options.py

@@ -663,6 +663,10 @@ def parseOpts(overrideArguments=None):
         '--autonumber-size',
         dest='autonumber_size', metavar='NUMBER',
         help='Specify the number of digits in %(autonumber)s when it is present in output filename template or --auto-number option is given')
+    filesystem.add_option(
+        '--autonumber-start',
+        dest='autonumber_start', metavar='NUMBER', type="int", default=1,
+        help='Specify the start value for the %(autonumber)s counter. Defaults to 1.')
     filesystem.add_option(
         '--restrict-filenames',
         action='store_true', dest='restrictfilenames', default=False,