use html5_polyglot docutils writer

[skurhse]

in rest2html:

https://github.com/github/markup/blob/46908504f5099b15a88030749f0aeb52cd2f6799/lib/github/commands/rest2html#L54

bumping the docutils writer to html5_polyglot would allow for text-level tags as so:

.. role:: alternative-voice
   :class: i, language-la

Lemonade consists primarily of :alternative-voice:`Citrus limon`.

It is not possible to use some html tags like i without it.