Add aggregate statistics support

[bensanmorris]

Google benchmark supports the display of aggregate stats (mean, median and standard deviation) if run with --benchmark_report_aggregates_only=[true|false] or --benchmark_display_aggregates=[true|false] and --benchmark_repetitions is specified with value > 1. Would be good to have these plotted on a benchmark's metric-specific chart.

An example output for aggregate stats:

---

Benchmark Time CPU Iterations

BM_poisson/8_mean 0.047 ms 0.047 ms 10
BM_poisson/8_median 0.047 ms 0.047 ms 10
BM_poisson/8_stddev 0.001 ms 0.001 ms 10
BM_poisson/16_mean 0.078 ms 0.074 ms 10
BM_poisson/16_median 0.068 ms 0.066 ms 10
BM_poisson/16_stddev 0.017 ms 0.012 ms 10
BM_poisson/32_mean 0.180 ms 0.180 ms 10
BM_poisson/32_median 0.178 ms 0.178 ms 10
BM_poisson/32_stddev 0.006 ms 0.006 ms 10
BM_poisson/64_mean 0.754 ms 0.752 ms 10
BM_poisson/64_median 0.737 ms 0.732 ms 10
BM_poisson/64_stddev 0.048 ms 0.047 ms 10
BM_poisson/128_mean 2.95 ms 2.95 ms 10
BM_poisson/128_median 3.00 ms 2.97 ms 10
BM_poisson/128_stddev 0.173 ms 0.204 ms 10
BM_poisson/256_mean 11.0 ms 11.0 ms 10
BM_poisson/256_median 11.1 ms 11.3 ms 10
BM_poisson/256_stddev 0.336 ms 0.391 ms 10
BM_poisson/512_mean 49.7 ms 49.7 ms 10
BM_poisson/512_median 49.7 ms 49.7 ms 10
BM_poisson/512_stddev 2.98 ms 2.84 ms 10