--custom-meta option to write specific information to tags, for --add-metadata

Syntax: --custom-meta "TAGNAME=string with optional %(format)s"
Example: --custom-meta "comment=%(webpage_url)s\n%(description)s"

This can be invoked multiple times for different tags.

Example 2:
 youtube-dl --add-metadata -f 18 --meta 'artist=%(uploader_id)s' --meta 'comment=url: %(webpage_url)s\nviews: %(view_count)s / Rating: %(average_rating)s\n\n%(description)s' https://www.youtube.com/watch?v=WzhW20hLp6M
(snip)
[ffmpeg] Adding metadata to 'Breaking Bad Remix (Seasons 3-5)-WzhW20hLp6M.mp4'
 mp4info Breaking\ Bad\ Remix\ \(Seasons\ 3-5\)-WzhW20hLp6M.mp4
(snip)
 Artist: uploader: placeboing
 Comments: url: https://www.youtube.com/watch?v=WzhW20hLp6M
views: 5601679 / Rating: 4.97684526443

Music and remixing by me.
Seasons 1 and 2 remix: http://www.youtube.com/watch?v=WsqdmqRgrIc
MP3 download: https://www.dropbox.com/s/39luqbbi5cu36du/Chris%20Lohr%20-%20Breaking%20Bad%20Remix%20seasons%203-5.mp3

youtube_dl/__init__.py

@@ -367,6 +367,7 @@ def _real_main(argv=None):
         'hls_prefer_native': opts.hls_prefer_native,
         'external_downloader_args': external_downloader_args,
         'cn_verification_proxy': opts.cn_verification_proxy,
+        'custommeta': opts.custommeta,
     }
 
     with YoutubeDL(ydl_opts) as ydl:

youtube_dl/options.py

@@ -707,6 +707,16 @@ def parseOpts(overrideArguments=None):
         '--add-metadata',
         action='store_true', dest='addmetadata', default=False,
         help='Write metadata to the video file')
+    postproc.add_option(
+        '--custom-meta',
+        action='append', dest='custommeta', default=[],
+        help='Write specific information to a metadata tag.'
+             'Syntax: "tagname=string to add with %(format)s" '
+             'The formatting syntax is the same as output. '
+             'Example: --custom-meta "comment=%(webpage_url)s\\n%(description)s" will '
+             'add a line with the url, then the description in the "comment" tag. '
+             'Tags are format-specific, common ones include: artist, comment, title, copyright, uploader. '
+             'This can be invoked multiple times for different tags.')
     postproc.add_option(
         '--metadata-from-title',
         metavar='FORMAT', dest='metafromtitle',

youtube_dl/postprocessor/ffmpeg.py

@@ -367,6 +367,10 @@ class FFmpegMetadataPP(FFmpegPostProcessor):
         if info.get('album') is not None:
             metadata['album'] = info['album']
 
+        for m in self._downloader.params.get('custommeta'):
+            key,val = m.split('=',1)
+            metadata[key] = val.replace('\\n', '\n') % info
+
         if not metadata:
             self._downloader.to_screen('[ffmpeg] There isn\'t any metadata to add')
             return [], info