data-jwplayer-* URL attributes are ignored in Moodle 3.3 onwards.

[kabalin]

As found while commenting on #5 It seems like data-jwplayer-* attributes are not retrieved properly for using in setup JSON object. e.g.
<a href="http://content.jwplatform.com/videos/3XnJSIm4-kNspJqnJ.mp4" data-jwplayer-subtitles="English: http://assets-jp.jwpsrv.com/tracks/2UEDrDhv.txt" data-jwplayer-description="some description">test subtitles</a>

is rendered as:
require(["media_jwplayer/jwplayer"], function(amd) { amd.setupPlayer({"playerid":"media_jwplayer_media_random5af1a61a44c1987","setupdata":{"playlist":[{"sources":[{"file":"https:\/\/content.jwplatform.com\/videos\/3XnJSIm4-kNspJqnJ.mp4"}],"title":"3XnJSIm4-kNspJqnJ.mp4"}],"width":400,"height":225},"logcontext":1618506,"logevents":["play","pause","complete"]}); });

[jproctor]

The embed code is looking for $options['htmlattributes'], but $options only has keys originaltext, trusted, and embedorblank, which are created in /filter/mediaplugin/filter.php near this comment:

// NOTE: Options are not passed through from filter because the 'embed'
// code does not recognise filter options (it's a different kind of
// option-space) as it can be used in non-filter situations.

That said, I think I know how to fix it. PR forthcoming.

[kabalin]

Great, thanks @jproctor!