feat: add yaml compiler (#509)

## 🧰 Changes

Adds a compiler for frontmatter.

If you try to save a doc in the editor with frontmatter, the save currently fails! Something like the following:

```
---
title: Getting started
---

Coming content
```

__tests__/flavored-compilers/yaml.test.js

@@ -0,0 +1,22 @@
+import { mdast, md } from '../../index';
+
+describe('yaml compiler', () => {
+  it('correctly writes out yaml', () => {
+    const txt = `
+---
+title: This is test
+author: A frontmatter test
+---
+
+Document content!
+    `;
+
+    expect(md(mdast(txt))).toBe(`---
+title: This is test
+author: A frontmatter test
+---
+
+Document content!
+`);
+  });
+});

processor/compile/index.js

@@ -13,3 +13,4 @@ export { default as rdmePinCompiler } from './pin';
 export { default as rdmeVarCompiler } from './var';
 export { default as tableCompiler } from './table';
 export { default as tableHeadCompiler } from './table-head';
+export { default as yamlCompiler } from './yaml';

processor/compile/yaml.js

@@ -0,0 +1,8 @@
+module.exports = function YamlCompiler() {
+  const { Compiler } = this;
+  const { visitors } = Compiler.prototype;
+
+  visitors.yaml = function compile(node) {
+    return `---\n${node.value}\n---`;
+  };
+};