
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