Support to pass graphviz output format

[sbrugman]

PDF is much crisper

Tested locally with:

impulse drawgraph --format pdf [my_package]

png:

pdf:

[seddonym]

Thanks for this, nice idea!

My concern with this is that I have been wondering about moving away from Graphviz as a dependency. By adding this to the API we are committing to supporting other file formats.

I'm going to leave this one open for the time being, until I've got a good understanding of whether there are any alternatives to Graphviz.

[sbrugman]

To make it more generic you could consider forwarding unknown options to the graph save/build methods: https://click.palletsprojects.com/en/5.x/advanced/#forwarding-unknown-options

This way there is no tight link with graphviz, but still there is the ability for passing arguments.