Allow custom reports

[bradwilson]

Today, we ship with several reports (XML v2, XML v1, CTRF, HTML, etc.), but don't really offer users a way to add their own reports.

Part of the complication is that the current design is set up to assume an input of our v2 XML, which isn't a full fidelity mapping of the entire object model. On the other hand, keeping the model in place for the full run might be unnecessarily expensive in terms of memory consumption. An alternative might be implementing reports as being able to intercept all messages in the pipeline (just like a runner reporter would today), except with the express purpose of building their own output models in real time rather than writing things to the console), and then at the end they would be given an opportunity to finalize their report as needed.