Consider adding a way to package for the current system

[TomaSajt]

This would be useful for people who want to build the app for their own system without having to worry about specifying the correct build task name.
Also, since linux+arm is not yet officially supported this would allow them to build the app without having to modify the mk script.

This could be achieved by conditionally setting the platform and arch arguments passed into electron-packager. Sadly, just using undefined or null didn't seem to work, so the solution might not be very pretty :/

Also, the 3rd party license copying logic might need to be updated too.
The .then() actually receives the output paths so you could use paths[0] for getting the directory instead of using _/${pj.name}/${pj.productName}-${x}-${y}