Shortcut for PDF direct output method.

This is useful because gerbv does not have much editing facility. You can delete objects and that's about all. I needed to manually edit some objects. So I use gerbv to export a PDF same as in gerber2graphtec and then edit the PDF manually in Inkscape. After saving the edited PDF this pull request allows sending it to the cutter by calling gerber2graphtec with the PDF filename instead of gerber. This simple change just detects if ".pdf" is supplied and skips the export step and converts the edited PDF instead.

This probably won't work with just any old PDf of course, but is useful for when a gerbv export needs manual edits. Inkscape doesn't seem to muck up the PDF to cause problems in my few tests.