Loopback and backward direction support

[decamel]

It would be greate if this library could support such structure of graph.

[nabil6391]

Can you share some usecase?

[decamel]

@nabil6391
Unfortunately I can not provide you a great example.
I was wondering how I can visualize Mealy machine state transitions and met some problems when I made next thing:

***
  @override
  void initState() {
    final node1 = Node.Id(1);
    final node2 = Node.Id(2);

    graph.addEdge(node1, node2);
    graph.addEdge(node2, node1);
  }
***

And also Mealy machine have some rules which do not changes state. In this case I had to make this:

    graph.addEdge(node1, node1);