Improvements to -spray and similar functionality

[johnwhitington]

Per customer suggestions:

• things like -retain-numbering are supposed to work for splitting and spraying. I'll make a note to fix the docs and fix any bugs in Cpdf relating to retention of bookmarks and page labels at the next release. Bookmarks work fine for -merge and -split (and -spray is really just -split in disguise....)
• I think "every nth page" is best done by extending (yet again!) the page range syntax. e.g: cpdf in.pdf "3-endSTEP4" or "3-end:4" -o out.pdf would select pages 3, 7, 11,...
• Implicit spray output names could be done with the same syntax as splitting: cpdf -spray-n 4 in.pdf -o %.pdf would create 1.pdf, 2.pdf, 3.pdf, 4.pdf...