Allow the functionality to shallow clone a graph

[alexhcheng]

We have a scenario where we have a "base" graph and then a number of enhanced graph which is just "base" graph + a few extra nodes and edges.

Instead of rebuilding the "base" graph each time, it would be nice to have the ability to clone this dictionary https://github.com/matiii/Dijkstra.NET/blob/master/src/Dijkstra.NET/Graph/Graph.cs#L12 and return a new Graph.

[alexhcheng]

@matiii : if you think this is a good idea, i can work on this. thanks