Override Vertex Id Mapping

I see the following code in `QuickGraph.Graphviz.GraphvizAlgorithm.Generate` which defines the `id` used for each vertex: 
```csharp
int i = 0;
foreach (TVertex v in this.VisitedGraph.Vertices)
    this.vertexIds.Add(v, i++);
```
It would be nice to be able to override the default 1, 2, 3, n to maybe something defined in `TVertex`. 