Add ability to embed subtitles in mkv files (closes #5434)

README.md

@@ -216,7 +216,7 @@ which means you can modify it, redistribute it or use it however you like.
     --recode-video FORMAT            Encode the video to another format if necessary (currently supported: mp4|flv|ogg|webm|mkv)
     -k, --keep-video                 Keep the video file on disk after the post-processing; the video is erased by default
     --no-post-overwrites             Do not overwrite post-processed files; the post-processed files are overwritten by default
-    --embed-subs                     Embed subtitles in the video (only for mp4 videos)
+    --embed-subs                     Embed subtitles in the video (only for mkv and mp4 videos)
     --embed-thumbnail                Embed thumbnail in the audio as cover art
     --add-metadata                   Write metadata to the video file
     --metadata-from-title FORMAT     Parse additional metadata like song title / artist from the video title. The format syntax is the same as --output, the parsed

youtube_dl/options.py

@@ -698,7 +698,7 @@ def parseOpts(overrideArguments=None):
     postproc.add_option(
         '--embed-subs',
         action='store_true', dest='embedsubtitles', default=False,
-        help='Embed subtitles in the video (only for mp4 videos)')
+        help='Embed subtitles in the video (only for mkv and mp4 videos)')
     postproc.add_option(
         '--embed-thumbnail',
         action='store_true', dest='embedthumbnail', default=False,

youtube_dl/postprocessor/ffmpeg.py

@@ -501,8 +501,8 @@ class FFmpegEmbedSubtitlePP(FFmpegPostProcessor):
         return cls._lang_map.get(code[:2])
 
     def run(self, information):
-        if information['ext'] != 'mp4':
-            self._downloader.to_screen('[ffmpeg] Subtitles can only be embedded in mp4 files')
+        if information['ext'] not in ['mp4', 'mkv']:
+            self._downloader.to_screen('[ffmpeg] Subtitles can only be embedded in mp4 or mkv files')
             return [], information
         subtitles = information.get('requested_subtitles')
         if not subtitles:
@@ -520,8 +520,9 @@ class FFmpegEmbedSubtitlePP(FFmpegPostProcessor):
             # Don't copy the existing subtitles, we may be running the
             # postprocessor a second time
             '-map', '-0:s',
-            '-c:s', 'mov_text',
         ]
+        if information['ext'] == 'mp4':
+            opts += ['-c:s', 'mov_text']
         for (i, lang) in enumerate(sub_langs):
             opts.extend(['-map', '%d:0' % (i + 1)])
             lang_code = self._conver_lang_code(lang)