Broken base64 images

[lambrechts]

First of all, thank you for developing this plugin. Made my life a lot easier.

I work with jupyter notebooks and mkdocs and there graphs will be outputted as base64 images. I discovered that Base64 images will be treated as a regular URL, and thus will be broken by the fix_image_src function. So I added a test for base64 images that checks if the url starts with data:image and ignores any base64 images found.

See PR #71

[timvink]

Released.. thanks again!

https://github.com/timvink/mkdocs-print-site-plugin/releases/tag/v2.3.4