Add IDs to sections

Done using this "elegant" replacement:
```js
source.replace(/(<h[1-6])>(.*?)(<\/h[1-6]>)/g, (_, p1, p2, p3) =>
  `${p1} id=${p2.replace(/<dfn.*?>/, '').replace('</dfn>', '').toLowerCase().replace(/\s+/g, '-')}>${p2}${p3}`)
```

Author: foolip