todo: create PDF specification with augmented Table of Contents

[dwarring]

Seems that this module could add to the table of contents, adding indices to the tables that (1) list tables (2) cross reference objects to tables & (3) cross reference entries to objects.

I'd have to modify the extraction to include references to each of the objects, also write code to extend the table of contents.

Would be generally handy for a range of PDF developers, who want to quickly find objects. Not easy to do with the current spec.