Improve output template (see desc)

* Objects can be traversed like `%(field.key1.key2)s`
* A number can be added to the field as `%(field+n)s`
* Deprecates `--autonumber-start`

yt_dlp/options.py

@@ -908,7 +908,7 @@ def parseOpts(overrideArguments=None):
     filesystem.add_option(
         '--autonumber-start',
         dest='autonumber_start', metavar='NUMBER', default=1, type=int,
-        help='Specify the start value for %(autonumber)s (default is %default)')
+        help=optparse.SUPPRESS_HELP)
     filesystem.add_option(
         '--restrict-filenames',
         action='store_true', dest='restrictfilenames', default=False,