Allow to run a satis build for one repo/package

Added an additional flag to generate the satis build for a single package. This helps to speed up the satis run if you have to deal with a lot of repos. In our case Jenkins will trigger the satis build after a new release was created. Since we have a lot packages (with a lot of versions) in our local satis repo, I was looking for a way to speed up the satis run.