stats: add percentile stats

[jqnatividad]

Gated by two options:

• --percentiles - boolean flag
• --percentile-list <arg> - a comma-delimited list of percentiles ranging from 0.0-100.0 inclusive. Compute the percentile using Nearest-Rank method (https://en.wikipedia.org/wiki/Percentile#The_nearest-rank_method).
  The default percentiles are: 5,10,40,60,90,95.

The result will be in the "percentiles" column, pipe(|)-separated (configurable with QSV_STATS_SEPARATOR env var, formerly QSV_MODES_SEPARATOR)

We choose these default values as --quartiles already covers q1 (25), q2 (50) and q3 (75).