Support <rp> tag

[gkatsev]

WebVTT supports ruby but it doesn't just defer to the HTML spec for ruby handling and specifies its own handling similar to HTML.
Supporting <rp> would make it easier to use ruby in WebVTT because vendors that support it could properly position the ruby above the text and hide the ruby parenthetical and those that don't support it will be just show everything.
Right now, an implementation without ruby support without adding some kind of parenthetical would mean that you may not be able to differentiate between text and ruby text and if a ruby parenthetical is added it'll either show up as part of the ruby text or as part of the text, while not bad, is not ideal.

Output of a ruby with vtt.js

Firefox native:

Chrome native:

Safari native:

(Ruby text borrowed from https://netflixtechblog.com/implementing-japanese-subtitles-on-netflix-c165fbe61989#124a)

[gkatsev]

There was recently an in-depth discussion whether <rp> should be deprecated from HTML whatwg/html#7590. Might be worth getting feedback from those folks into adding <rp> support.