Save images offline, in the snapshot (#770)

* Implemented image restore from rr_dataURL

* Implement saving images in the snapshot

* Fixed image saving, added a test

* Rename data-src to data-rrweb-src

* Updated the guide

* Rename recordImages to inlineImages and try catch

packages/rrweb-snapshot/test/html/picture.html

@@ -4,5 +4,6 @@
       <source type="image/webp" srcset="assets/img/characters/robot.webp" />
       <img src="assets/img/characters/robot.png" />
     </picture>
+    <img src="/images/robot.png" alt="This is a robot" />
   </body>
 </html>