Creating DataURI for image inlining

[niko-dunixi]

Should solve the issue for this preprocesssor and create a better
user-experience. Also enables other downstream renderers to work
agnostically and not only the html renderer.

• Preprocessor output files are deleted when hbs renderer is called (from mdBook > 0.3.1) rust-lang/mdBook#1087

[niko-dunixi]

The one thing I was not really able to fully test and what I'd like some guidance on, is how we can test the text/plain as an image src. From what I can tell the text/plain is a fully valid data uri option

What I don't know if it will embed as I expect when placed into an image tag.

Currently I'm not sure how you get plantuml to generate text instead of an svg. Perhaps there is a bug, but using plantuml,txt instead of just plain plantuml didn't actually generate text output.

I can revert to the original method already in use for embedding text if this doesn't actually work as expected, but I just need to know how to generate those text documents.

[niko-dunixi]

AH! It was my mistake. I somehow read plantuml,txt instead of plantuml,format=txt. I'll restore that branch of functionality.