Support YAML front matter in Markdown files

Summary

vscode-yaml should apply schema validation to YAML front matter in markdown files.

Description

YAML is often prepended to markdown files to provide structured metadata. This "front matter" pattern is commonly implemented by static site generators, including Hugo, Gatsby, Jekyll and Nikola. YAML Front Matter precedes all Markdown content. The file may or may not start with ---.

VScode recognizes YAML front matter in markdown files and formats it appropriately as long as there is not an initial --- (implemented here).vscode-yaml does not recognize YAML front matter, even when extension references are configured correctly.

vscode-yaml does support multiple YAML files delimited by ---, see #43. This works with or without an initial ---.

Reproduction

(1) Configure settings.json to use the same schema for YAML and Markdown files:

{
    "editor.minimap.enabled": false,
    "yaml.format.enable": true,
    "yaml.format.proseWrap": "always",
    "yaml.schemas": {
        "./my_schema.json": "*.yml",
        "./my_schema.json": "*.md"
    },
    "git.autofetch": true
}

(2) Create a YAML file as test.yml:

title: "A Blog Post"
date: 2019-07-07
draft: true

(3) Confirm that the vscode-yaml applies the schema to test.yml.

(4a) Create a Markdown file test.md with the same YAML contents in as front matter:

title: "A Blog Post"
date: 2019-07-07
draft: true
---
This is content of my blog post, which will be processed as Markdown.

(4b) Add an initial --- and save as test_initial_delimiter.md:

---
title: "A Blog Post"
date: 2019-07-07
draft: true
---
This is content of my blog post, which will be processed as Markdown.

(5) Confirm that vscode-yaml does not validate the schema of the YAML in either markdown file.

Begin with the schema mapping in settings.json and compare test.yml with test.md and test_initial_delimiter.md. Trace the Markdown front-matter handling and verify that the configured schema validates both delimiter variants without treating the Markdown body as YAML.