Series enhancement for unit prefix

[Low-power]

This change fix and add followings:

• Correct prefix names of IEC binary prefixes that prevously used in the program (ex. M -> Mi, G -> Gi).
• Add support of using SI metric prefixes (power of 1000 instead of 1024).

Option -u and -U handling has been changed to support select binary or metric prefix, as following format:
[-u h|b|k|m|g|H|B|K|M|G[i|s]] [-U h|b|k|m|g|H|B|K|M|G[i|s]]

The optional second character could be used in argument of -u and -U, that specfiying IEC (i) or SI (s) prefix. Since nload has previously always use binary prefixes for data calculation, IEC binary prefix is the default.

The help message has been updated accordingly.

For example to use human readable metric prefix with bit for data rate, and gibi binary prefix with byte for total amount of data, run:
nload -u hs -U Gi

[Low-power]

The latest commit added some additional larger units as requested in #13.

[jknoxdev]

LGTM!