Custom variables for PDF certificates

In PDF certificates various variables from the course learning resource context and the user  course / assessment context can be used. In some cases it would be handy to have custom variables somewhere. E.g. when the title of the certificate is not the course title but rather the title of a certain test.

Today a custom certificate must be created. A simple solution is to add custom variables that can be set in the course configuration. 

$custom1
$custom2
$custom3

When set empty or only whitespace, the variable is removed. 

The variables must be added to all the PDF workers (Phantom, PDFforms etc.)