Remove trackers by default

[Nikoli]

For tracking purposes google adds unique fields at container level to mp4 files downloaded from youtube:

$ mediainfo -f _video.mp4 |grep '^g'
gsst                                     : 0
gstd                                     : xxxxxx
gssd                                     : yyyyyyyyyyyyyyyyyyyyyyyyyyy
gshh                                     : zzzzzzzzzzzzzzzz.googlevideo.com

https://superuser.com/questions/503868/how-i-can-remove-all-google-data-from-youtube-video-downloads

I think youtube-dl should by default create reproducible downloads: if several people from different countries using different platforms execute at different time same youtube-dl commands for same urls they should get exactly the same files. Now md5sum for mp4 files from youtube changes after simple redownloading file from same system and location, because gs* entries are changed.

The most obvious and trivial workaround is to simply remux:
avconv -i _video.mp4 -c copy -flags bitexact video.mp4.

-flags bitexact is required, because else info about ffmpeg/libav versions will be added to file

$ mediainfo -f video.mp4 |grep -i lavf
Writing application                      : Lavf55.12.0

P.S. With matroska files bitexact option works fine too.