Add support for <sub> in markdown

[machour]

Currently <GithubHtmlView /> lacks support for the <sub> HTML tag.

For example:

_{This sentence is rendered as really small on github, but not in gitpoint}.

If anyone wants to tackle this issue, I will gladly provide help 🎉

[AkshayIyer12]

I want to work on this issue.

[andrewda]

@AkshayIyer12 Thanks for the interest! I think we already have #629 open. It still needs some changes, though, so feel free to take a look at that code!