[youtube] Add two-factor account signin (TOTP only)

Additional work is required to prompt the user for the SMS or phone call codes, as there is no framework currently to prompt the user during an extraction operation.

Fixes #3533

youtube_dl/__init__.py

@@ -314,6 +314,8 @@ def parseOpts(overrideArguments=None):
             dest='username', metavar='USERNAME', help='account username')
     authentication.add_option('-p', '--password',
             dest='password', metavar='PASSWORD', help='account password')
+    authentication.add_option('-2', '--twofactor',
+            dest='twofactor', metavar='TWOFACTOR', help='two-factor auth code')
     authentication.add_option('-n', '--netrc',
             action='store_true', dest='usenetrc', help='use .netrc authentication data', default=False)
     authentication.add_option('--video-password',
@@ -748,6 +750,7 @@ def _real_main(argv=None):
         'usenetrc': opts.usenetrc,
         'username': opts.username,
         'password': opts.password,
+        'twofactor': opts.twofactor,
         'videopassword': opts.videopassword,
         'quiet': (opts.quiet or any_printing),
         'no_warnings': opts.no_warnings,