[Shock] - Allow for multiple template engines to run on a single source file

[matthew-carroll]

Make it possible to specify a series of template engines/conversions to run on source content.

Here's an example from Lume as to why this is useful:

A typical example is a file using Markdown to render HTML but Nunjucks to insert variables or includes. To do that, you can use an array to add several engines:

---
title: My post
templateEngine: [vto, md]
---

# Hello, this is the post title {{ title }}