The output now contains items per second instead of time in seconds.

Also, two new options are introduced:
* The `--output` option allows you to specify the output file.
* The `--output_format` option means the format for the printed output.
  The default is "console". It prints rows proceeded per second to the
  stdout. The "json" format contains all the information about the
  benchmark in a format similar to Google Benchmark's.

Usually, these options should be used together to dump machine-readable
results for the benchmarks.

NO_DOC=perf test
NO_CHANGELOG=perf test
NO_TEST=perf test