feature: Render Markdown with inline PlantUML

[sangxxh]

Problem:

It'd be very powerful if md-to-pdf can also render inline PlantUML diagrams when converting .md to .pdf

Solution:

Use a PlantUML parser to convert text within uml or plantuml code block to SVG image and render them along with the PDF:

```plantuml
@startuml
:Hello world;
:This is defined on
several **lines**;
@enduml

[simonhaenisch]

Interesting idea, but I'd rather not implement it in the core (since there's many other diagram syntaxes)... instead, marked is pretty simple to extend, so I assume it would be pretty straightforward to inject a plantuml parser.

I'll think about how I can expose extending the parser, and keep you updated. I'm working on a new major release though (v3), so this feature will only come along with that release.

[hadrien-toma]

@simonhaenisch is the parser exposed now? Can you provide an example on how to extend it? This sounds very interesting! :heart_eyes:

[simonhaenisch]

No I don't think it's exposed yet (only options which includes the renderer) but I'm open to PRs for that 🤓 (I don't have much free time at the moment)

[PeterDaveHello]

This will be a great feature, would love to see it very much!