Formatter: Preserve Frontmatter when formatting documents

[marcoroth]

This pull request adds support for preserving frontmatter when formatting HTML+ERB documents.

When the first Node within the DocumentNode is a HTMLTextNode and starts/ends with --- we treat that HTMLTextNode as frontmatter content and preserve/format it exactly as-is.

Example

The following document is now being preserved:

---
title: My Page
layout: application
published: true
---

<div class="container">
  <h1><%= @title %></h1>
</div>

Previously it was formatted as:

--- title: My Page layout: application published: true ---

<div class="container">
  <h1><%= @title %></h1>
</div>

Thanks to @joshuap for the nudge!