Add a `spawnOptions` key to `options.child` that will be passed as the `options` argument of `child_process.spawn` when creating a spooky instance. The default is the empty object. Note that setting `options.child.spawnOptions.env` will pass those variables *instead* of the environment of the node process. It is recommended to make sure that `PATH` and friends are passed. close #72
Also factor child spawning out as a separate module for tests
Accept npm's format for package.json so --save is less disruptive in the future.