simple command line tool for local execution

Hey there,
I understand the benefit of running cwlviewer on a webserver so that anybody can use the tool without installation. However, I think for most people it would be much more convenient to provide a simple command line tool that can be used to produce such overview image locally. I would appreciate something like this:
`cwlviewer -wf my_workflow.cwl -img workflow_overview.png`

I think that workflow visualization should be an integral part of every workflow execution. With such a command line utility, one could automatically execute cwlviewer for every workflow run and store the generated overview image as part of the experiment's documentation together with the results.

In the current form, cwlviewer is only of limited use for me since my workflows are often created on the fly and I don't want to upload them to a public git repository every time.

CWL viewer is a great tool, I would really appreciate your help on this.

Review the existing cwlviewer workflow-visualization path and determine how a local command-line entry point could reuse it. Use the requested `cwlviewer -wf my_workflow.cwl -img workflow_overview.png` invocation as the behavior to validate; done means a local CWL workflow produces the requested overview image without uploading it to a public repository.

Written by the indexing model from the issue text.