Traversal method closure should take 'from' and 'to' as arguments

[moaxcp]

This will allow the user to lookup the edge between the two vertices and modify it.

preOrder('A') { from, to ->
    from.fillcolor = 'green'
    to.fillcolor = 'green'
    edge(from.key, to.key) {
        color = 'green'
    }
}