Support paragraphs inside Blockquote

[pestevez]

This issue comes from https://github.com/Automattic/facebook-instant-articles-wp/issues/912

The Blockquote element in Instant Articles supports paragraphs (and perhaps other HTML elements) but I could not generate a set of rules to support the following output:

<blockquote>
  <p>First paragraph</p>
  <p>Second paragraph</p>
</blockquote>

The documentation for the blockquote mentions that it has Text, however I manually created an article with paragraphs inside the blockquote and it was rendered correctly (in different lines.)