Background colour

[modem7]

Heya,

Thanks for this project! I've been using it on and off for a few years to show how Docker infrastructure can work to others!

By adding [bgcolor=colour]; to the graph syntax, it's possible to make the diagrams a little less retina scarring white.

e.g.

// Docker Network Graph
graph {
	graph [splines=true] [bgcolor=black];

Is a custom background colour something that can be readily implemented at all?