Fixes #39: more compact output for html and markdown

[lukstafi]

For html, we just remove class="". We could do better (there's sometimes redundant <div>), but it's tricky with the types.

For markdown, we output one less endline after </details>. It's justified because separators for vlist and tree will themselves move to a new line.

[lukstafi]

@c-cube friendly ping?

[c-cube]

Thanks, that looks good :)