support HTML definition lists (`<dl>`, `<dt>`, and `<dd>`)

[chrispy-snps]

Support HTML definition lists, such as:

<dl>
  <dt>term 1</dt>
  <dd>
    <p>This is a definition of term 1.</p>
  </dd>
  <dt>term 2</dt>
  <dd>
    <p>This is a definition of term 2.</p>
  </dd>
</dl>

The widely-used Pandoc utility supports definition lists:

https://pandoc.org/MANUAL.html#definition-lists

using the PHP Markdown Extra syntax:

https://michelf.ca/projects/php-markdown/extra/#def-list