Add ability to serialize data flow graph

[curran]

Maybe something like

ReactiveModel.serializeGraph()

[curran]

Depends on datavis-tech/graph-data-structure#9

[curran]

Part of reactive-model 1.0 #31

[curran]

Necessary for #32

[curran]

Woohoo now looks like this

{
  "nodes": [
    {
      "id": "fullName"
    },
    {
      "id": "firstName"
    },
    {
      "id": "lastName"
    }
  ],
  "links": [
    {
      "source": "firstName",
      "target": "fullName"
    },
    {
      "source": "lastName",
      "target": "fullName"
    }
  ]
}

[curran]

• Add documentation