Add margins config and allow for multiple PDF pages

[andjsch]

This PR allows to set margins for a page.

Previously, if the generated content was longer than the page, it content would have been cut off. This PR allows to set margins for a PDF page and content to not be cut off - resulting in multiple pages if needed.

Feel free to add the following to the addons config file:

/**
 * The margins applied to PDF pages.
 */
'margins' => [
    'top' => 0,
    'right' => 15,
    'bottom' => 5,
    'left' => 10,
    'unit' => 'mm',
],