Add support to obfuscate images

Actual Behavior

The addon only obfuscates text

Expected Behavior

It should obfuscate images because they can also contain sensitive informations.

Additional Notes

• Get all images in a document: https://developer.mozilla.org/en-US/docs/Web/API/Document/images
• It should cover both <img> and CSS or inline style="background-image:*
• It should obfuscate any images added to the DOM when navigating the page. For this a MutationObverser can be used.