Add support for the <track> tag -- for subtitles / captioning

[kjcole]

I often caption my videos using VTT captions. T'would be nice if the video directive supported the <track> tag to make use of it. Here's an example taken from W3 Schools:

<video width="320" height="240" controls>
  <source src="forrest_gump.mp4" type="video/mp4">
  <source src="forrest_gump.ogg" type="video/ogg">
  <track src="fgsubtitles_en.vtt" kind="subtitles" srclang="en" label="English">
  <track src="fgsubtitles_no.vtt" kind="subtitles" srclang="no" label="Norwegian">
</video>