Add support for child.spawnOptions

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