Add support for MDX

[zenorocha]

Inspiration:

• https://www.joshwcomeau.com/react/wonderful-emails-with-mjml-and-mdx/#composing-emails-with-mdx
• https://www.useplunk.com/markdown-to-email

Components needed:

• https://github.com/zenorocha/react-email/issues/53
• https://github.com/zenorocha/react-email/issues/52
• https://github.com/zenorocha/react-email/issues/49
• https://github.com/zenorocha/react-email/issues/48
• https://github.com/zenorocha/react-email/issues/42
• https://github.com/zenorocha/react-email/issues/13

[vcapretz]

Assigned myself for this one, going to work on it!