support parse c4 models

[highkay]

the c4 models is very popular in software architecture stage. I suggest to support parse c4 models in startleft.

[highkay]

https://github.com/plantuml-stdlib/C4-PlantUML

[dantolin-iriusrisk]

Hi, @highkay! Thanks for your suggestion! I have already escalated the C4 proposal to consider it as a future new processor. I will keep you updated about whether it is planned to be developed and when. Anyway, you can also consider the option of doing it by yourself following this guide.