Add `mj-include` component

[IanEdington]

Hey this is a pretty simple ask.

We are in the process of migrating from mjml to mjml-react.

We're planning on doing it slowly over the next couple months and would need a mj-include tag in order for it to work properly.

Is there any reason this wasn't included in the library?

[IanEdington]

I ran into some issues with relative paths and ended up defining filePath directly in the mjml options.

If there's interest I can put up a PR

[daliusd]

PRs are always welcome.

[daliusd]

As this project is not maintained anymore and nothing happened with this issue I am closing it.