Allow to assign multiple template engines to a format.

[oscarotero]

It's possible to assign multiple template engines to a page, using the templateEngine value. For example:

---
title: Hello
templateEngine: njk,md
---

# {{ title }}

But it's not possible to register a format to use multiple template engines. For example:

site.engine([".njk.md"], nunjucksEngine, markdownEngine);

This would allows to bring a initial mdx support with:

site.engine([".mdx"], jsxEngine, mdEngine);