Using --self-contained video track src should be converted to data-URI

[jronallo]

If I have the following video markup (that happens to be in a Markdown file), I expect the src of the track element to be converted to a data-URI. Currently the

Please let me know if you need a test case and I can make one in a gist.

<video id="tubthumping" poster="slides/assets/video/tubthumping.png">
  <source src="slides/assets/video/tubthumping.mp4" type="video/mp4">
  <source src="slides/assets/video/tubthumping.webm" type="video/webm">
  <track label="karaoke captions" kind="captions" srclang="en" src="slides/assets/video/tubthumping.vtt" default>
</video>

Expected html:

<video id="tubthumping" poster="data:img/png;base64,AAAA...AAA4">
  <source src="data:video/mp4;base64,AAAA...AAA4" type="video/mp4">
  <source src="data:video/webm;base64,AAAA...AAA4" type="video/webm">
  <track label="karaoke captions" kind="captions" srclang="en" src="data:text/vtt;base64,AAAA...AAA4" default>
</video>

Actual:

<video id="tubthumping" poster="data:img/png;base64,AAAA...AAA4">
  <source src="data:video/mp4;base64,AAAA...AAA4" type="video/mp4">
  <source src="data:video/webm;base64,AAAA...AAA4" type="video/webm">
  <track label="karaoke captions" kind="captions" srclang="en" src="slides/assets/video/tubthumping.vtt" default>
</video>