JsonBenchmarkConsumer Default Path

[vjkoskela]

The JsonBenchmarkConsumer implementation should support a reasonable default configuration. Namely, the output directory should default to something reasonable, for example, ./target/site/perf. Next, the default output file name should take on the name of the class. The class should continue to support overriding the path, which is especially useless when used outside Maven. I am less sure about continuing to support of the file name.