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