Support example html in comments and add API to access to Section

Example comment with html:

```scss
// A basic button.
//
// :hover    - The hover state
// :active   - The pressed state
// :disabled - The disabled state
//
// <button class="m-btn $modifier-class">Button</button>
//
// Styleguide 1.1
```

Accessible via `Section#example_html`.