Preserve Vorbis Comment vendor string when copying streams

[hdante]

Summary of the bug:
Hello, by default ffmpeg does not preserve the vendor string in formats that use vorbis comment. Is is possible to do so ? Could it be implemented ?

How to reproduce:

$ ffmpeg -i Best_of_the_Beast.flac -c copy ffmpeg.flac
$ ffmpeg -i Best_of_the_Beast.opus -c copy ffmpeg.opus
$ mediainfo Best_of_the_Beast.flac | grep library
Writing library                          : libFLAC 1.3.2 (UTC 2017-01-01)
$ mediainfo Best_of_the_Beast.opus | grep library
Writing library                          : libopus 1.2.1
$ mediainfo ffmpeg.flac | grep library
Writing library                          : Lavf58.12.100
$ mediainfo ffmpeg.opus | grep library
Writing library                          : Lavf58.12.100

ffmpeg version 4.0 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 8.1.0 (GCC)
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-avresample --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libass --enable-libbluray --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-nvenc --enable-omx --enable-shared --enable-version3
  libavutil      56. 14.100 / 56. 14.100
  libavcodec     58. 18.100 / 58. 18.100
  libavformat    58. 12.100 / 58. 12.100
  libavdevice    58.  3.100 / 58.  3.100
  libavfilter     7. 16.100 /  7. 16.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  1.100 /  5.  1.100
  libswresample   3.  1.100 /  3.  1.100
  libpostproc    55.  1.100 / 55.  1.100

Patches should be submitted to the ffmpeg-devel mailing list and not this bug tracker.

[hdante]

Any comments here ?