Implement confidence level selection for summary stats.

[Pavel-Durov]

This allows users to specify the confidence level for the summary statistics.
Example of haste diff output:

$ haste diff -c 99 1 0
confidence level: 99%

 Benchmark             Datum1 (ms)   Datum0 (ms)  Ratio  Summary           
 permute/lua/1000      8483 ±  597   7882 ±  292   0.93  indistinguishable 
 bounce/lua/1500       9458 ±  381   9054 ±  192   0.96  indistinguishable 
 list/lua/1500         8758 ±  552   8512 ±  381   0.97  indistinguishable 
 richards/lua/100     42362 ±  484  41613 ± 1391   0.98  indistinguishable 
 deltablue/lua/12000  13889 ±  555  13665 ±  302   0.98  indistinguishable 
 nbody/lua/250000      4649 ±  401   4604 ±  100   0.99  indistinguishable 
 json/lua/100         16726 ±   85  16623 ±  165   0.99  indistinguishable 
 cd/lua/250           39655 ±  757  39490 ±  175   1.00  indistinguishable 
 mandelbrot/lua/500    1308 ±    3   1307 ±    4   1.00  indistinguishable 
 sieve/lua/3000        4160 ±   52   4174 ±   34   1.00  indistinguishable 
 storage/lua/1000     23806 ±  100  23894 ±  124   1.00  indistinguishable 
 queens/lua/1000       5114 ±  290   5160 ±  169   1.01  indistinguishable 
 towers/lua/600        9377 ±  348   9517 ±  810   1.01  indistinguishable 
 havlak/lua/1500      78254 ± 1153  82589 ±12269   1.06  indistinguishable 

$ haste diff -c 95 1 0
confidence level: 95%

 Benchmark            Datum1 (ms)  Datum0 (ms)  Ratio  Summary           
 permute/lua/1000      8483 ± 454   7882 ± 222   0.93  indistinguishable 
 bounce/lua/1500       9458 ± 290   9054 ± 146   0.96  indistinguishable 
 list/lua/1500         8758 ± 420   8512 ± 290   0.97  indistinguishable 
 richards/lua/100     42362 ± 368  41613 ±1058   0.98  indistinguishable 
 deltablue/lua/12000  13889 ± 422  13665 ± 230   0.98  indistinguishable 
 nbody/lua/250000      4649 ± 305   4604 ±  76   0.99  indistinguishable 
 json/lua/100         16726 ±  65  16623 ± 126   0.99  indistinguishable 
 cd/lua/250           39655 ± 576  39490 ± 133   1.00  indistinguishable 
 mandelbrot/lua/500    1308 ±   2   1307 ±   3   1.00  indistinguishable 
 sieve/lua/3000        4160 ±  39   4174 ±  26   1.00  indistinguishable 
 storage/lua/1000     23806 ±  76  23894 ±  94   1.00  indistinguishable 
 queens/lua/1000       5114 ± 221   5160 ± 129   1.01  indistinguishable 
 towers/lua/600        9377 ± 265   9517 ± 616   1.01  indistinguishable 
 havlak/lua/1500      78254 ± 877  82589 ±9335   1.06  indistinguishable 
pd@bencher16 ~/yk-benchmarks main ~ 9ada704646f9c85fea307722806cfd30ce084831

$ haste diff -c 90 1 0
confidence level: 90%

 Benchmark            Datum1 (ms)  Datum0 (ms)  Ratio  Summary           
 permute/lua/1000      8483 ± 381   7882 ± 186   0.93  7.08% faster      
 bounce/lua/1500       9458 ± 243   9054 ± 122   0.96  4.27% faster      
 list/lua/1500         8758 ± 353   8512 ± 243   0.97  indistinguishable 
 richards/lua/100     42362 ± 309  41613 ± 888   0.98  indistinguishable 
 deltablue/lua/12000  13889 ± 354  13665 ± 193   0.98  indistinguishable 
 nbody/lua/250000      4649 ± 256   4604 ±  64   0.99  indistinguishable 
 json/lua/100         16726 ±  54  16623 ± 106   0.99  indistinguishable 
 cd/lua/250           39655 ± 483  39490 ± 112   1.00  indistinguishable 
 mandelbrot/lua/500    1308 ±   2   1307 ±   3   1.00  indistinguishable 
 sieve/lua/3000        4160 ±  33   4174 ±  22   1.00  indistinguishable 
 storage/lua/1000     23806 ±  64  23894 ±  79   1.00  indistinguishable 
 queens/lua/1000       5114 ± 185   5160 ± 108   1.01  indistinguishable 
 towers/lua/600        9377 ± 222   9517 ± 517   1.01  indistinguishable 
 havlak/lua/1500      78254 ± 736  82589 ±7835   1.06  indistinguishable 

[vext01]

LGTM