Add pages to pdf_overlay_stamp()

[trevorld]

• Adds a pages argument to pdf_overlay_stamp().

• By default if left missing uses all pages.

• Implementation in R and C++ closely matches how the pages argument for pdf_rotate() is implemented.

• Note the qpdf cli overlay operation supports three different types of page ranges: to, from, repeat

  • https://qpdf.readthedocs.io/en/stable/cli.html#overlay-underlay
  • Our pages corresponds to their notion of to and indicates which pages in the input file should have the stamp overlayed. This use of pages seems to match how pages is used in pdf_rotate().

• We consolidate the pages argument documentation R/qpdf.R for all three functions that use it. Note the pages documentation for pdf_subset() was previously quietly getting dropped in favor of the pages documentation for pdf_rotate().

• I've manually looked at some test pdf output (including those defined in tests/testthat/test_overlay.R) and it seems to be working as intended.

closes #29