Add CSV output reporting.

It's too late and I'm not entirely happy with `CSVReporter` and especially `CSVReporterTest` but the CSV feature is too cool to wait. Would appreciate improvements.

Pretty much passed on `PluginTest.groovy` entirely. This needs tackling separately.

Can't wait to get these CSV data files into R and start making some plots!
So this is executed before `beforeExecute` then, right?