[Shock] - Support pure Yaml pages

[matthew-carroll]

Currently we support two types of pages. Markdown pages through the Markdown plugin, and Jinja pages through the Jinja plugin.

Markdown pages are a collection of variables at the top of the file in a Yaml syntax known as Frontmatter, and then all of the actual Markdown syntax in the file comes after that, and is placed in a content property.

Sometimes it's preferable to define an entire page in terms of Yaml properties without any Markdown at all.

For example, consider a webpage that describes a fragment shader. The page has a title, description, and a code block. Each page only has a few things that are allowed to be different. There's no control over generic content. Therefore, these pages can be defined in terms of a set of Yaml properties alone. But there's currently no plugin to find and load Yaml files as pages.