WIP: Allow saving graphs as images

[devdanzin]

Add the ability to save the output of wily graph as an image. It requires installing Kaleido.

This is WIP, would need a new test and adding a new dependency to pyproject.toml, which I'm willing to do if the PR is to be accepted. Also the help string could be better.

Supported formats: png, jpg, webp, svg and pdf.

This addresses part of #92.