Show role name in Efsm graph name

[fangyi-zhou]

Currently we have a default of digraph G, better to replace that with Role@Proto

[fangyi-zhou]

/cc @zakcutner

[fangyi-zhou]

https://github.com/backtracking/ocamlgraph/blob/1c028af097339ca8bc379436f7bd9477fa3a49cd/src/graphviz.ml#L573

It seems that the graph name is fixated to G, I'll think about another way to add the info in the dot output...