[coub] Support for saving video tags

[SebiderSushi]

Checklist

• I'm reporting a site feature request
• I've verified that I'm running youtube-dl version 2019.08.13
• I've searched the bugtracker for similar site feature requests including closed ones

Description

On the site coub.com an uploader can associate tags with any uploaded video. These tags can be used on the site to filter or search for videos. Apart of that, they often contain valuable information about a video such as the music used, the sources of the video material (i.e. what show/movie/etc. the footage was taken from) or persons, figures or characters appearing in the video. Since coub.com does not provide the ability for uploaders to redact a video description, tags fill that role on coub.com, at least as seen from the users perspective.

Currently, when running youtube-dl --write-info-json <coub URL> none of the tags can be found inside the file, which is where i'd expect them.

It would be great if youtube-dl could maybe include the tags as an array in the info json file, for later use by a program, e.g. for offering the same filter/sort ability from the site in offline archives.
It might also be a good idea to construct and use a tag list whenever the options --add-metadata or --write-description are given. This would be useful to any regular user who doesn't want or need a complete info json file but might want to preserve the additional information mentioned above with no need to interact with the data using any script or program. Since there is no such thing as a video description on coub.com this might also be safe to do without any collisions.

Example URLs (on the site, tags are shown as the grey bubbles located under the video):

https://coub.com/view/13o4xm
https://coub.com/view/1ptni6
https://coub.com/view/1pu7i9
https://coub.com/view/1sr59n

https://coub.com/view/1ztcpw
https://coub.com/view/vyrqm
https://coub.com/view/1zy9nv
https://coub.com/view/1q744d

Second block of examples is for videos with no tags - make sure youtube-dl doesn't trip over these.
Alternatively to these links just check the feed on the coub.com frontpage for examples.