Goldmark extension for markdown sub-headers.
## Title
## Subtitle
text
## Subtitle 1
## Subtitle 2
text
<header>
<h2>Title</h2>
<p>Subtitle</p>
</header>
<p>text</p>
<h2>Subtitle 1</h2>
<h2>Subtitle 2</h2>
<p>text</p>
// add sub-headers support to golmark markdown parser
md := goldmark.New(subheaders.Option)