Ref: #10625
In a strict sense, <track>s with kind=captions are not subtitles. [1]
openload misuses this attribute, and I guess there will be more
examples, so I add it to common.py.
Also allow extracting information for subtitles-only <video> or <audio>
tags, which is the case of openload.
[1] https://www.w3.org/TR/html5/embedded-content-0.html#attr-track-kind