YAML 前置内容解析扩展（Markdig 规范文档）

添加了对 YAML 前置内容解析的支持：

YAML 前置内容丢弃

如果存在前置内容，它将不会被渲染：

---
this: is a frontmatter
---
This is a text
.
<p>This is a text</p>

但如果前置内容不在第一行，它将被解析为常规 Markdown 内容

This is a text1
---
this: is a frontmatter
---
This is a text2
.
<h2>This is a text1</h2>
<h2>this: is a frontmatter</h2>
<p>This is a text2</p>

它期望精确的三个破折号 ---：

----
this: is a frontmatter
----
This is a text
.
<hr />
<h2>this: is a frontmatter</h2>
<p>This is a text</p>

它可以以三个点 ... 结束：

---
this: is a frontmatter

...
This is a text
.
<p>This is a text</p>

如果结束前置内容标记（... 或 ---）不存在，那么 --- 将被渲染为 <hr>：

---
this: is a frontmatter
This is a text
.
<hr />
<p>this: is a frontmatter
This is a text</p>

它期望恰好三个点 ...：

---
this: is a frontmatter
....
This is a text
.
<hr />
<p>this: is a frontmatter
....
This is a text</p>

前置内容在遇到包含三个点 ... 或三个破折号 --- 的行时结束：

---
this: is a frontmatter
 ....

Hello
---
This is a text
.
<p>This is a text</p>

允许在引导字符后存在空格：

---
this: is a frontmatter
...
This is a text
.
<p>This is a text</p>

允许在尾随字符后存在空格：

---
this: is a frontmatter
...
This is a text
.
<p>This is a text</p>