Support pdflatex

[koppor]

Ideas:

1. Use pythontex and the remote PlantUML server using Python's plantuml.
2. Use pythontex and its bash functionality to locally execute PlantUML.
   • This would also solve Introduce caching #2
   • Issue: Unclear how I get latex counters into the bash code: https://tex.stackexchange.com/q/419102/9075
3. Build on checklistings, which generates a checklistings.sh file for the compiler. Drawback: Users have to call an external tool.
4. Use bashful to execute bash scripts.
5. Use hvextern

The package download can download files, but this does not help much here as we can also download plantuml within a bash script.

[koppor]

Alternative: Align with the plantuml.sty project.