feat: add flag for unhandled promise rejections (#1278)

Author: kumavis

src/raven.js

@@ -426,7 +426,11 @@ Raven.prototype = {
    */
   _promiseRejectionHandler: function(event) {
     this._logDebug('debug', 'Raven caught unhandled promise rejection:', event);
-    this.captureException(event.reason);
+    this.captureException(event.reason, {
+      extra: {
+        unhandledPromiseRejection: true
+      }
+    });
   },
 
   /**