Implement `markdown_in_html_blocks` extension about commonmark-hs

[not-my-profile]

Ah thanks, I didn't know that. I tried:

<figure>
*test*
</figure>

apparently with CommonMark you need a newline after the start tag. The following works as expected:

<figure>

*test*
</figure>

[jgm]

Commonmark has its own built in way of including markdown in HTML blocks, so this is not needed (or desired---it's a mess to handle arbitrary markdown in HTML blocks).

[jgm]

Yes, you need the blank lines. (Better to include one before the closing tag, too, though it isn't strictly needed.)