Add functionality to add link annotations

[mcgouganp]

Link annotations are useful for creating constructs like Table of Contents or Indexes.

This adds a new function pdf_add_link which makes a section of a page clickable, and clicking takes the viewer to a nominated destination in the document.

Signed-off-by: Paul McGougan paul@plmservices.com.au

[AndreRenaud]

Thanks for this, it's a great addition.