Allow use of local d3 distribution

[Rubyfi]

I'd be interested in a feature that allows users to load d3.js locally instead of from unpkg (similar to loading mermaid.js locally).
Of course, I'd be happy to contribute this change myself.

Is there a dependency to a specific version of d3 or is it just the latest by default?
I'd go with allowing to specify a desired version via a new config flag d3_version and basically use the same approach that is used to load mermaid.js.

[mgaitan]

this would be great. go ahead!