Add offset argument for page numbering

[widdowquinn]

Many PDFs that I deal with (e.g. PhD theses) have lead-in pages that are not numbered. The pdfcomments page numbering can then be out of sync with the document itself, which can be confusing. For example:

$ pdfcomments test.pdf
$ tail -n 40 test.txt
p186: The methodology of this needs to be explained somewhere. What is the relationship being used?
p187: How were these identified?

may refer to pages numbered 179 and 180 in the document.

This could be remedied with a command-line option --offset that takes the number of pages as an argument, and offsets the page numbering in the output file, so that numbers match the page numbers of the document, e.g.

$ pdfcomments test.pdf --offset 7
$ tail -n 40 test.txt
p179: The methodology of this needs to be explained somewhere. What is the relationship being used?
p180: How were these identified?

[michaelmhoffman]

I have been thinking about a more capable page renumbering option, which is described in #9. But that can wait—I would accept a pull request with implementation of a --page-numbering=start option. If the PhD theses you get are set up properly, implementation of #8 might help you even more.