Patch: DCSubtitle support (not supported by FFmpeg)

[forthrin]

FFmpeg does not support converting of the DCSubtitle format used on at least on site (Viafree), causing subtitle conversion to fail with an error.

$ youtube-dl --verbose --write-sub --sub-lang no --convert-subtitles srt -o "%(title)s.%(ext)s" https://www.viafree.no/programmer/underholdning/black-widows/sesong-1/episode-1
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'--verbose', u'--write-sub', u'--sub-lang', u'no', u'--convert-subtitles', u'srt', u'-o', u'%(title)s.%(ext)s', u'https://www.viafree.no/programmer/underholdning/black-widows/sesong-1/episode-1']
[debug] Encodings: locale UTF-8, fs utf-8, out utf8, pref UTF-8
[debug] youtube-dl version 2019.04.30
[debug] Python version 2.7.16 (CPython) - Darwin-16.7.0-x86_64-i386-64bit
[debug] exe versions: ffmpeg 4.1.3, ffprobe 4.1.3, rtmpdump 2.4
[debug] Proxy map: {}
[Viafree] episode-1: Downloading webpage
[debug] Using fake IP 84.213.87.216 (NO) as X-Forwarded-For.
[mtg] 701057: Downloading video JSON
[mtg] 701057: Downloading streams JSON
[mtg] 701057: Downloading m3u8 information
[mtg] 701057: Downloading f4m manifest
[debug] Default format spec: bestvideo+bestaudio/best
[info] Writing video subtitles to: Black Widows - Ep. 1.no.xml
[debug] Invoking downloader on u'https://mtgxno04-vh.akamaihd.net/i/open/20xxxxxx/2026xxxx/202689xx/20268968/e1ac784ca167cf95ece3e93d9b2c23b1_20268968_,225,512,908,1400,2000,3500,.mp4.csmil/index_5_av.m3u8?null=0&id=AgBU0TVjVNANPPa80lwKzXdzDVFusH50SozUjT4RobYP%2frCDnYnwex6JiYwTALm707%2fo9Vtao82XwQ%3d%3d&hdntl=exp=1557401206~acl=%2f*~data=hdntl~hmac=11477aa66190af94b722fa651537fd2d46b635e35e40242cf3e92a5089f2bf15'
[download] Black Widows - Ep. 1.mp4 has already been downloaded
[download] 100% of 1.03GiB
[debug] ffmpeg command line: ffprobe -show_streams 'file:Black Widows - Ep. 1.mp4'
[ffmpeg] Fixing malformed AAC bitstream in "Black Widows - Ep. 1.mp4"
[debug] ffmpeg command line: ffmpeg -y -loglevel 'repeat+info' -i 'file:Black Widows - Ep. 1.mp4' -c copy -f mp4 '-bsf:a' aac_adtstoasc 'file:Black Widows - Ep. 1.temp.mp4'
[ffmpeg] Converting subtitles
[debug] ffmpeg command line: ffmpeg -y -loglevel 'repeat+info' -i 'file:Black Widows - Ep. 1.no.xml' -f srt 'file:Black Widows - Ep. 1.no.srt'
ERROR: file:Black Widows - Ep. 1.no.xml: Invalid data found when processing input
Traceback (most recent call last):
  File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 2054, in post_process
    files_to_delete, info = pp.run(info)
  File "/usr/local/bin/youtube-dl/youtube_dl/postprocessor/ffmpeg.py", line 638, in run
    self.run_ffmpeg(old_file, new_file, ['-f', new_format])
  File "/usr/local/bin/youtube-dl/youtube_dl/postprocessor/ffmpeg.py", line 239, in run_ffmpeg
    self.run_ffmpeg_multiple_files([path], out_path, opts)
  File "/usr/local/bin/youtube-dl/youtube_dl/postprocessor/ffmpeg.py", line 235, in run_ffmpeg_multiple_files
    raise FFmpegPostProcessorError(msg)
FFmpegPostProcessorError: file:Black Widows - Ep. 1.no.xml: Invalid data found when processing input

[dstftw]

FYI, if you want such patches to be reviewed and merged you should file a PR. Also you must provide clean explanation of what it supposed to fix.