Helper script for emulating a duplex scan

[maciex]

First idea of the script was to separate each document and then unite it, smth similar to this:

pdfseparate "$file_A" $TMP_DIR/scan-separated-%d-A.pdf
pdfseparate "$file_B" $TMP_DIR/scan-separated-%d-B.pdf

pdfunite $TMP_DIR/scan-separated-*-*.pdf "$output_file"

The script assumed odd pages and even pages are scanned in ascending order.
So before a scan one would need to change the order of all the pages to get a proper scan document... but we can scan the sheets in descending order and then reverse using some tools.

After some research on PDF tools that are able to split/merge/reverse the order of the pages in the PDF... the "script" boils down to one command:

qpdf --collate --empty --pages odd.pdf even.pdf z-1 -- output.pdf

I wrapped the command into a helper script and added a short README mention.

Fixes #40