wrap progress in object

So data could be extended in the future

README.md

@@ -18,7 +18,7 @@ const geometry = new THREE.TorusGeometry(20, 10, 30, 30);
 const slicer = new SLICER.Slicer();
 slicer.setGeometry(geometry);
 const gcode = await slicer.slice(settings)
-  .progress(({ done, total, action }) => {
+  .progress(({ progress: { done, total, action } }) => {
     const percentage = `${(done / total * 100).toFixed()}%`
     console.log(action, percentage);
   });

example/save.js

@@ -17,7 +17,7 @@ jsonLoader.load('models/airplane.json', async geometry => {
 
   const slicer = new Slicer().setGeometry(geometry);
   const gcode = await slicer.slice(settings)
-    .progress(({ done, total, action }) => {
+    .progress(({ progress: { done, total, action } }) => {
       const percentage = `${(done / total * 100).toFixed()}%`
       document.write(`<p>${action}, ${percentage}</p>`);
     });

src/slicerWorker.js

@@ -6,7 +6,7 @@ const loader = new THREE.JSONLoader();
 const onProgress = progress => {
   self.postMessage({
     message: 'PROGRESS',
-    data: progress
+    data: { progress }
   });
 }