Markdown reader:  small efficiency improvement.

Switched `notFollewdBy' rawHtmlBlocks` ->
`notFollowedBy' (htmlTag isBlockTag)`, which is more
efficient.
Markdown parser:  avoid backtracking when closing `</div>` not found.
Markdown reader: Improved parsing of nested divs.

Formerly a closing div tag would be missed if it came right
after other block-level tags.