Add support for named input files

[rocketraman]

pdftk supports named input files e.g.:

pdftk A=foo.pdf B=bar.pdf cat A B output foobar.pdf

See https://www.pdflabs.com/docs/pdftk-man-page/#dest-input-files

[vog]

Note that the "cat" operation is not yet implemented in mcpdf. Does it still make sense to support named input files? What's your exact use case?

[rocketraman]

My main use case is indeed the cat operation. The named inputs are useful when one wants to do interesting things e.g.:

pdftk A=foo.pdf B=bar.pdf cat A1-2 B1-2 A3-end B3-end output foobar.pdf