[#82614560] GraphSON visualization

As part of the GraphSON visualization, we create this module which can create a Graphviz graph from a Gremlin graph.

We could extend this module to allow decorating the graph with Gremlin attributes and labels.  This would be accomplished by extending the options object.

FYI Direction happens to be one of the classes that is already available as gremlin.Direction (line 57 of gremlin.js).