Support YAML front matter

It is very common to write Markdown files that include YAML front-matter at the top of the file, like this [Jekyll](https://jekyllrb.com/) example:

```
---
layout: page
title: About
permalink: /about/
nav: 0
---
```

When running `markflow` against a file that includes the above, it gets reformatted to

```
----------------------------------------------------------------------------------------
layout: page title: About permalink: /about/
nav: 0
------
```

and Jekyll is unable to render the file.