progressstream: add stats func

src/progress-stream.js

@@ -14,6 +14,11 @@ class ProgressStream extends TransformStream {
         this._interval = null;
     }
 
+    stats() {
+        const totalSecs = Math.round((Date.now() - this._startTime)/1000);
+        return { startTime: this._startTime, totalSecs, transferred: this._transferred };
+    }
+
     _start() {
         this._startTime = Date.now();
         this._started = true;