Merge Consecutive Blockquotes in Markdown

Older versions of Hugo use a library named Blackfriday to convert Markdown to HTML, which is not compliant with CommonMark (unlike Goldmark in newer versions of Hugo). One problem with Blackfriday is that blank lines inside blockquotes are treated as part of the blockquotes. For example,

> One > Two

will be converted to:

<blockquote> <p>One</p> <p>Two</p>
</blockquote>

With the CommonMark specs, it should be converted to two blockquotes …