Allow HTML tags inside <mj-preview>

[dbackeus]

While migrating from mjml to mrml we hit an error on <mj-preview> with HTML content:

<mjml>
  <mj-head>
    <mj-preview>
      <b>bold here</b> not bold here
    </mj-preview>
  </mj-head>
</mjml>

This works in mjml but raises unexpected token at position 42..44 error in mrml due to the presence of the <b> tag.

The documentation of mj-preview does not mention any particular restriction of contents.