Fix multiple-character handles

This PR implements a non-BC change to support older versions. With this fix, newer versions can handle up to 702 files whereas users with an older version of pdftk are limited to 26 files.

Generated file handles: A, B, ..., Z, AA, AB, ..., ZZ

We should probably add a small note in the code or README about the fact that older versions of pdftk are unable to handle many files.