Support for plantuml

[SwedishChef]

If the markdown file has plantuml sections like
```plantuml
@startuml
...
@enduml
```
is it possible to configure a plantuml server or path to jar file to get this rendered to the pdf?

[vb64]

I don't use Java and won't be able to test if it works correctly when using a jar file.

The pure python lib and external server option seems more preferable.

As recommended by Google:

pip install plantuml

from plantuml import PlantUML
import os

# Create a server object to call for your computations (uses the public server by default)
server = PlantUML(url='http://www.plantuml.com/plantuml/img/')

uml_code = """
@startuml
Alice -> Bob: Hello Bob
Bob --> Alice: Hi!
@enduml
"""

# Render directly from content and save to a file
output_filename = "output.png"
server.processes(uml_code, outfile=output_filename)

print(f"Diagram saved to {os.path.abspath(output_filename)}")

[vb64]

Implemented in version 1.11

More details here. Your feedback is welcome.