Implement a FaunusGremlinStepRegistry for custom steps

[jschott780]

We have implemented our own faunus gremlin steps to augment the current faunus gremlin library. The steps encapsulate a handful of in-house ETL and analytic algorithms. While integrating our custom steps we did not see a clean way to register our custom steps in FaunusPipeline. It would be an incredibly useful feature if we could specify in the faunus props a class that would register custom steps.