Support networkx node-link JSON format

[dhimmel]

networkx supports a graph serialization format it calls node-link data that can be read/written to JSON. See

• networkx.readwrite.json_graph.node_link_data
• networkx.readwrite.json_graph.node_link_graph

While this format doesn't appear to be a standard, it is simple and interoperable due to its JSON compatibility. It supports node & edge attributes.