Add 'id=' option to headings for anchoring

Hi,

if there was a 'id' option at the heading of a groups I could reference directly to the grouped pages via anchoring.

inc/pagequery.php / line 172 may look like this:

```
$render .= '<h' . $level . ' style="' . $indent_style . '" id="' . $heading . '">' . $heading . '</h' . $level . '>' . DOKU_LF;
```

Maybe you could add this in upcoming versions.

Thnx,

Floek
