present: fix Markdown bugs
present: fix Markdown bugs

The handling of subsubsections was not completely right,
causing unexpected subsubsubsections (#### inside ##)
to go into an infinite loop. Handle that.

Also, my usage of goldmark's (not completely documented)
SetAttributeString was wrong. Need []byte, not string.

Change-Id: Ib127a72b94b5a46adc9047fdb88dd2a8d03e73fe
Reviewed-on: https://go-review.googlesource.com/c/tools/+/223601
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools