This also improves Commonmark coverage, e.g. it fixes tests
HTML_blocks_ex179_2894..2906 and Lists_ex308_5439..5457.
In other words, we go from 271 out of 652 to 273 out of 652.
From the commonmark spec:
A list is loose if any of its constituent list items are separated by
blank lines, or if any of its constituent list items directly contain
two block-level elements with a blank line between them. Otherwise a
list is tight. (The difference in HTML output is that paragraphs in a
loose list are wrapped in <p> tags, while paragraphs in a tight list are
not.)
