TinkerGraph currently works with gryo, graphml and graphson - all internal formats to TinkerPop. This could easily be extended to work with any format implementing the appropriate interfaces (i.e. also external third-party formats) by allowing for the `gremlin.tinkergraph.graphFormat` setting for TinkerGraph to be set to the existing three settings, but also allow for it to be the fully qualified class name for a Io.Builder<I extends Io> interface. TinkerGraph could then dynamically instantiate this class (which we can expect to have a zero-arg constructor) and use it to load/save data. This might actually also clean up some of the existing code around load/save.