feat: threshold for moving up a prefix

[BPplays]

the current handling for prefixes seems to go to the next one up if the lower one is >= then 1000 e.g. 1435 bytes -> 1.4 kilobytes i want some option like --prefix-threshold 10000 to be like: 1435 bytes -> 1435 bytes 14350 bytes -> 14.3 kilobytes 1435 kilobytes -> 1435 kilobytes 10000 kilobytes -> 10 megabytes 14350 kilobytes -> 14.3 megabytes