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>
```
Override it so that `HeaderSerializer` is call by hand, as pegdown doesn't use plugins for SuperNode like HeaderNode (https://github.com/sirthias/pegdown/blob/cc862769ba6906258d8d2321dd2dd0ceb5d7348f/src/main/java/org/pegdown/ToHtmlSerializer.java#L353).