Markdown filter does not handle HTML block-like text with empty lines

*Created by: Kuro Kurosaka *

(This was branched from the issue 685) DirectShape.md has a

tag, tags directly under tag with many empty lines within the and tags. and appear twice after the xlf file is merged.

Originally it was thought the cause of this is same as the issue 685. It is partly true but there are differences. So I made a new issue.

In issue 685, the tags are parsed by FlexMark and represented as an HtmlInline node. In this case, a fragment of HTML expression (e.g. "

\n \n This cell has is") is represented as an HtmlBlock. Although it is categorized as HtmlBlock, it is not well formed.

It was considered to use the same fix strategy for issue 685. But that would not work. This is because the Markdown spec allows markdowns within the HTML construct as seen in http://spec.commonmark.org/0.28/#example-120 We can't just put together all HTML-like pieces of text together and send it to HTML-subfilter.

We may need to reconsider the design decision of using HTML subfilter to fully support the Markdown spec.