add option for less-strict html blocks

[3b]

cl-mongo README.md embeds documentation created with docmentation template, which doesn't close <p> tags, and the embedded chunks are <p> followed immediately by a <blockquote> in 1 html block rather than separated into 2 as I understand the markdown docs to require.

Github parses these chunks as HTML blocks when rendering documentation, but doesn't seem to in issues.

Probably can get reasonable parsing by optionally allowing multiple html-block-in-tags in one html-block, and adding a variant of <p> that is closed by a html-block-in-tags (or possibly only a subset of block tags?) rather than </p>