Provide ability to add RunListeners using SPI (#80)

@dsaff for the limited case of using `ServiceLoaderWrapper` to load listeners, it wouldn't be that hard to have `RunNotifier` call the current listeners notifying them about the new listeners. I had thought that you wanted some kind of notification when any service was loaded; if so, I don't see a good way to do that other than stdout, stderr and the JDK logger.