Provide ability to add RunListeners using SPI (#80)

I think as soon as we commit this, people will want to use `ServiceLoader` to enable other extension points, yes. You had requested a mechanism for reporting loaded classes, but I'm not sure how we would do that in the general case. Maybe we require that you pass a `RunNotifier` to the `ServiceLoaderWrapper` and we hope anywhere where we want to load classes we have a `RunNotifier`