Fix SI units calculation and introduce OptionUseIECUnits()

[edigaryev]

Problem: currently progressbar uses SI units by default:

progressbar/progressbar.go

Line 1069 in 3d12297

sizes := []string{" B", " kB", " MB", " GB", " TB", " PB", " EB"}

...but calculates them with base 1024, which is not correct and should be 1000 instead.

It's also not possible to choose which the units to use, e.g. the github.com/dustin/go-humanize package, for example, allows one to either use Bytes() or IBytes().

Solution: fix SI unit base and introduce OptionUseIECUnits() option to enable IEC units like KiB, MiB, etc.

[edigaryev]

@schollz would you mind taking a look at this?

[schollz]

It looks like tests are failing?

[edigaryev]

TestBarSmallBytes: use proper SI units

Indeed, fixed in ba1eaf1.

[schollz]

Great, thanks!!