format user counter as timing information

Hi,

I would like to add some more timings to the output, so I added some user counters, and in the function to be benchmarked, I record some timing information for certain block of code.

The output looks like:

```
--------------------------------------------------------------------------------------------------------------------------
Benchmark Time CPU Iterations cor eig reg
--------------------------------------------------------------------------------------------------------------------------
spectra/10/40/2/2/process_time/real_time 0.012 ms 0.012 ms 63176 1.56285m 7.6555m 994.525u
spectra/10/40/2/4/process_time/real_time 0.016 ms 0.017 ms 41991 1.55633m 0.0126054 1018.47u
spectra/10/40/2/6/process_time/real_time 0.022 ms 0.022 ms 32986 1.56102m 0.0183076 1019.17u
spectra/10/40/2/8/process_time/real_time 0.028 ms 0.028 ms 25630 1.5661m 0.0236718 1021.9u
spectra/10/40/2/10/process_time/real_time 0.034 ms 0.034 ms 20381 1.53995m 0.029994 997.325u
```

As it shows, the custom counter values are shown as counts, which are not intuitive. I think it would be good to introduce a flag to format the counter as timing, and output in the same format and unit as the Time and CPU section.