Support mj-include #37

Hi, while integrating gomjml into a mail templating workflow, I noticed that mj-include does not seem to be supported.

I could not find any mention of mj-include in the documentation, and I also could not find support for it in the source code.

This is a blocker for a common MJML use case: splitting large templates into reusable partials such as:

• shared header
• shared footer
• repeated content blocks
• different email variants built from the same layout

For example, a template like this:

<mjml>
  <mj-body>
    <mj-include path="header.mjml" />
    <mj-section>
      <mj-column>
        <mj-text>Hello</mj-text>
      </mj-column>
    </mj-section>
    <mj-include path="footer.mjml" />
  </mj-body>
</mjml>