Add anchor while rendering HTML header

When markdown header like `# Header` is encountered, renders it in HTML including an anchor `<h1><a name="header">Header</a></h1>`.

It makes easier to have link pointing to specific documentation section.

Anchor is generated by normalizing header node content (children). For example:

```
# Header 1
##   % An header 2 with not alphanum char like § or ⇒
```

... is rendered in HTML as following:

```html
<h1><a name="header-1">Header 1</a></h1>
<h2><a name="--an-header-2-with-not-alphanum-char-like---or--">% An header 2 with not alphanum char like § or ⇒</a></h2>
```
Generate normalized anchor from header content (children)