Support for triggering the rendering of pumla diagrams with PlantUML

[DrMarkusVoss]

Is your feature request related to a problem? Please describe.
Currently pumla does not support the rendering of diagrams. You have to do it manually.

Describe the solution you'd like
It would be cool if it is possible to call something like
pumla render
which would make pumla call PlantUML on all pumla diagrams (of the input folder) and create PNGs in the output folder. Of course, in order to do so, pumla must know the installation place of the PlantUML library or it must be on the global path.