nested field lists in custom directives

G'day,

Firstly, thanks to everybody who has worked on ReST/docutils, you're
awesome.

I'm trying to create a custom directive for videos (my only output is
html), and I would like to be able to include tracks (subtitle files)
within the directive.

For those unfamiliar with html5 video, the output looks something like this:
<video autoplay>
<source src="foo.ogg">
<track kind="subtitles" src="foo.vtt" srclang="en">
</video>

What I'd like is a directive with nested field lists. eg:
.. video:: sources
    :autoplay: True
    :track: :src: foo.vtt
               :srclang: en
               :kind: subtitles
    :controls: false

I saw nested lists like this somewhere in the sandbox (though now I can't
find it again), but I think it was just a suggestion rather than an
actually implemented directive. If this is possible, how would it work with
option_spec in the directive definition? Alternatively is there a better
way of solving this problem?

Thank you,
Robin.

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