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!
Might be cool to have something like Jake's dynamic fixture environments so we could even run those tests in parallel: `https://github.com/JakeWharton/sdk-manager-plugin/blob/master/src/test/java/com/jakewharton/sdkmanager/TemporaryFixture.java`