Packages for all, not just for Munki

This adds a new option - `-m` or `--munki-import` - which when passed will import the package into Munki. The default behaviour is to just output a package in the current working directory.

I've also removed the option for `--install-time` as the postinstall now "does the right thing" depending on what type of volume it's installed to (booted or unbooted).