Feature: Option to bind only a part of the pdf

[FazinHan]

As the title says, please add an option to select page numbers from the input pdf, and treat that as the pdf for any further binding.

[momijizukamori]

Thanks, I meant to file this and forgot. UI-wise, I think we should take a leaf from standard printer dialogs, where the pages are defined as a list of comma-separated numbers, and can be ranges. eg, 1-4,7,10-12 is pages 1, 2, 3, 4, 7, 10, 11, 12

Edge cases we should figure out how to handle - specifications that aren't in original order (ie, 10-12,7,4), and ranges with no end (ie, 10-,7,4). I'm inclined to say either show the user an error or treat it as a single-page specification for the second one, but the first one is slightly more complicated and it may be worth investigating how the PDF lib handles page selection