CLI support of import and export operation

[drmoeller]

Is your feature request related to a problem? NO, just to an idea:
I thought about an option to make Domain Storytelling results consumable to a wider audience, also to the people involved in such sessions, but also to others. Without the need to export results as pictures and upload them to some remotely accessible backend by hand; but to make use of versioned .egn files, e.g. within a CI pipeline, by feeding them to Egon.io CLI for export as a picture for upload (or availability in GitLab Pages and the equivalent on GitHub, for example). In an automated fashion.

Describe the solution you'd like
You are already offering a Container Image of Egon.io. What's missing from my point of view is an option to launch it like a CLI, something like

egon export --egn-file ./foo.egn --format png --output ./foo.png

Additional context
The idea of using this in the context of CI pipelines.