Librarify the analysis process

[japaric]

Currently criterion always performs the analysis of the benchmark in the same way. Criterion needs to be librarified to let the users configure the analysis process.

[mstewartgallus]

Would this mean I could add a custom statistic such as the number x where 99% of the time statistics will tend to be less than x (which in the case of a normal distribution is mean + 3 * standard-deviation?)