Add init command (or add postinstall and postupdate hooks)

[michalkvasnicak]

This command will simple define build, start, test to scripts section of package.json in your project.

[michalkvasnicak]

Perhaps we should install commands in postinstall or postupdate hooks instead of command. It's hard to run binary using npm. There isn't an equivalent for yarn spust ...

[michalkvasnicak]

Hooks should add scripts to package.json only if there aren't already. In this case it should just output an information.