Change image data handling mechanism

[nilskruthoff]

Right now images are processed to compressed base64 strings and included in the Markdown output like [image1.jpg](data:image/png;base64, <base64 content>). We want to change the following:

• include a function to manually handle the base64 data rather than including them in the Markdown directly
• • for that we introduce a ImageExtractionMode enum to the ParserConfig