Support for edge modifiers

[AnjoMan]

It would be valuable to be able to mark an edge with a modifier. E.G. in the below graph, the edge (n2, n3) has an arrow modifier which could be used to make a directed graph, or to modify the significance of the edge in some other way.

     n1---------n2
                ^
                |
                n3

We already support edge labels such as n1---(label_1)---n2 but this change would allow the user to have modifiers and labels on the same edge

``n1<---(edge_3)--->n2`