Variable number of percentiles for pt-query-digest

New option --percentiles. This will permit gathering info about multiple percentiles other than the default 95% for the different collected query parameters.
e.g: pt-query-digest --percentiles 80,95,98 <slow_log_file>
example output:
# Attribute pct total min max avg 80% 95% 98% stddev median
# ============ === ======= ======= ======= ======= ======= ======= ======= ======= =======
# Count 100 17744
# Exec time 100 8848554s 1s 1000s 499s 793s 918s 964s 281s 487s
# Lock time 100 871522s 0 99s 49s 76s 93s 93s 28s 47s