How to set bold to fields inside preservetext()?

Hi.

In this case
```php
$footer = $section->addFooter();
$footer->addPreserveText('Page {PAGE} of {NUMPAGES}', static::FONT_DEFAULT, static::LINE_RIGHT);
```
the result will be:  
Page 1 of 1

How to make the numbers bold?  
Page **1** of **1**
