Add support for the `cat` operation

[rocketraman]

Add support for the cat operation. See https://www.pdflabs.com/docs/pdftk-man-page/#dest-op-cat

See also issue #4.

[itsme-410]

I agree. This is the most important feature for me and pretty much the only reason I use pdftk. Thanks though for the project!

[Ramblurr]

Yup, another +1 for cat.

This is also the feature I use the most, to extract subsets of pages from a document.

Example:

pdftk input.pdf cat 2-end output output.pdf
pdftk input.pdf cat 3-4 output output.pdf
etc

[vog]

Any volunteers for working on that?

[MikhailKlemin]

Now, when Fedora droped pdftk, that would be really useful...