Use page labels instead of page index+1

[michaelmhoffman]

Currently, pdfcomments prints out page labels before each comment that are always the page index plus 1. It would be better if page labels defined within the document would be respected.

See the PDF 1.7 standard (32000-1:2008), section 12.4.2, "Page Labels", for the distinction between the fixed integer page index and the more flexible page label.