Re-introduce onclone option (Fix #1434)

src/Clone.js

@@ -611,14 +611,21 @@ export const cloneWindow = (
                 documentClone.documentElement.style.left = -bounds.left + 'px';
                 documentClone.documentElement.style.position = 'absolute';
             }
+
+            const result = Promise.resolve([
+                cloneIframeContainer,
+                cloner.clonedReferenceElement,
+                cloner.resourceLoader
+            ]);
+
+            const onclone = options.onclone;
+
             return cloner.clonedReferenceElement instanceof cloneWindow.HTMLElement ||
             cloner.clonedReferenceElement instanceof ownerDocument.defaultView.HTMLElement ||
             cloner.clonedReferenceElement instanceof HTMLElement
-                ? Promise.resolve([
-                      cloneIframeContainer,
-                      cloner.clonedReferenceElement,
-                      cloner.resourceLoader
-                  ])
+                ? typeof onclone === 'function'
+                  ? Promise.resolve().then(() => onclone(documentClone)).then(() => result)
+                  : result
                 : Promise.reject(
                       __DEV__
                           ? `Error finding the ${referenceElement.nodeName} in the cloned document`

src/index.js

@@ -17,6 +17,7 @@ export type Options = {
     ignoreElements?: HTMLElement => boolean,
     imageTimeout: number,
     logging: boolean,
+    onclone?: Document => void,
     proxy: ?string,
     removeContainer: ?boolean,
     scale: number,