Fix incorrect json output from input
Fix incorrect json output from input

graph {
  a
  subgraph X {
    subgraph Y {
    }
  }
}

and improve pretty-printing.

ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz