Support symbols

[silby]

the html rendering is identical to what djoths and thus pandoc do: you get a <span class="symbol">contents</span>. There's an argument for making this a knob and just rendering the symbols as plain text if not requested but I'm not here to add knobs.

PR contains an unrelated tweak to the test helpers which you are free to drop.

[lpil]

Thank you!

Thanks for figuring out how we should render this as HTML. I think your approach of following pandoc makes a lot of sense 👍

I've left a note inline RE the parsing technique

[silby]

Repushed, thanks.

[lpil]

Thank you, couple more notes inline.

[silby]

Repushed. I still am not confident I understand what you are asking for re: iteration.

[silby]

This should be ok now I think.

[lpil]

Thank you!!