Update package.json mac build option (#2401)
Update package.json mac build option (#2401)

Hi, I'm proposing adding macOS build option into the package.json

This follows the pre-existing convention where they have specified individual builds for windows / linux.

Personally, I spent ~8mins trying to work out how to build to mac only, ended up running `package-linux`, `package-all`, `package`. Then popped over to https://www.electron.build/configuration/configuration#configuration, and finally decided that adding `package-mac` might have been better.

Thanks =)
chentsulin/electron-react-boilerplate,chentsulin/electron-react-boilerplate