Request: Add preexisting PDF

[oligneisti]

Description of feature/request

Add a preexisting pdf to the exported pdf file, for instance at the start of the document or a the end.

This feature would be used to

For many people it would be useful to add a pdf cover page. It could also be used to add all sorts of pages that need to be strictly formatted.

Btw. wonderful plugin. The best I've seen in this field.