Shift placement ot multistamp/multiwatermark by several pages

[vsenko]

A possible implementation of #733.

The idea is to be able to configure the Watermark for how many pages have to be skipped before applying multistamp/multiwatermark.

My understanding of the internals of PDF is not that good, thus I suspect that this implementation is quite brute force.

[hhrutter]

Hello!

Thanks for the effort.
Generally I expect a conversation before any PR, to understand and see if there is really a need.

This makes sense.
Let me think about a good fit into the API.

[hhrutter]

Thanks for your time and input!

This is part of https://github.com/pdfcpu/pdfcpu/releases/tag/v0.6.0

I have elaborated on your idea and pdfcpu now allows for specifying both the start page # of the stamp file and also the first page number of the destination file to apply multi stamping.

Whenever possible any extension of a pdfcpu command also needs to be reflected on the CLI.
The config syntax for PDF stamp has been extended therefore. See pdfcpu help stamp.