Add an option to format Dot graphs with Graph attributes

[s1gtrap]

I needed a dot graph with a transparent background, and you can add attributes for nodes/vertices with Dot::with_attr_getters, so I was surprised to learn graph attributes weren't supported. Obviously there's only a single graph being formatted at a time, so adding a getter is a bit overkill. I made a petgraph::dot::Config variant with a key/value String do, and figured it might as well be submitted upstream.