Allow different output formats for stats

[qinsoon]

We currently output stats in harness_end as tab-separated values. We should support different human-/machine-readable output formats for stats

[caizixian]

I think a relevant PR is #823

It would be useful for us to have a format to capture more than just end of benchmark stats. There are probably per-GC stats, etc. that are worth capturing in a structured way so that we can script the analysis without parsing logs