Add --mark-watched feature (Closes #5054)

youtube_dl/options.py

@@ -170,6 +170,10 @@ def parseOpts(overrideArguments=None):
         action='store_const', dest='extract_flat', const='in_playlist',
         default=False,
         help='Do not extract the videos of a playlist, only list them.')
+    general.add_option(
+        '--mark-watched',
+        action='store_true', dest='mark_watched', default=False,
+        help='Mark videos watched (YouTube only)')
     general.add_option(
         '--no-color', '--no-colors',
         action='store_true', dest='no_color',