Handle layout key in frontmatter

After some discussion with @timneutkens and @jxnblk we think it makes most sense to allow layout to be optionally specified in frontmatter.

---
title: Awesome Page
slug: /awesome
layout: ./PageLayout
team:
  - jxn
  - mrmrs
  - johno
---

Resulting in, when layout is defined, the result would be something similar to:

import PageLayout from './PageLayout'

// ... stuff

export default PostLayout(frontmatter)(Markdown)

Something to consider: Should a layout be a HOC or standard component?

[timneutkens]

Regarding layout being a HOC vs normal component, I would currently prefer a HOC because it gives maximum flexibility on top level pages.

[johno]

This functionality can now be supported with export syntax.