Add missing collate flag if generating labels via glabels-3-batch

Port patch originally added as a PR to https://github.com/jimevins/glabels/pull/45

Currently it seems the glabels-3-batch is missing the option to toggle collate flag that can be set when using GUI. I frequently need to generate dozens of pages of labels from a CSV file and then separate individual pages in such a way that each page contains labels of exactly one type. To use collation on a sheet of 4 labels so that one label is on one page, 4 copies and collation are needed: -c 4 -a 1

New flag is --collate (-a), default 0, can be switched on. Tried to compile and can confirm the new parameter works as expected (using Linux Mint 64bit).