support custom metadata

[ilka-schulz]

PDFs can hold the standard info dictionary and a custom metadata dictionary.

I can define them in LaTeX with hyperref (see section 5.9 "Option pdfinfo" in the manual).

pdfinfo can then read those meta data out again, man pdfinfo.1 says:

NAME
       pdfinfo - Portable Document Format (PDF) document information extractor (version 3.03)

SYNOPSIS
       pdfinfo [options] [PDF-file]

[...]
OPTIONS
[...]
       -custom
              Prints custom and standard metadata.
[...]

As far as I can tell, pdf-rs does not recognize the custom meta data dictionary yet. I would love a feature which allows to read those meta data.