Support of <header> and <footer> elements

Support of <header> and <footer> elements which are rendered on each page. Footer additionally supports special span tags "pageCounter" and "totalPages" which are replaced by the current page number or the total page count (see example below).

```
<footer>
    <div style='text-align:center;'>Page <span class="pageCounter"></span>/<span class="totalPages"></span></div>
</footer>
```