Add support for custom progress providers

[jh0x]

Add support for custom progress provider in form of lambda/funtction:

auto p = ProgressBar([&]{ return getValue(); }, {.out = &hide, .speed = 1, .show = false});
//                   ^^^^^^^^^^^^^^^^^^^^^^^^^

[coveralls]

Pull Request Test Coverage Report for Build 12494646274

Details

• 38 of 38 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 99.109%

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Totals
Change from base Build 12267205189:	0.04%
Covered Lines:	445
Relevant Lines:	449

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💛 - Coveralls

[jh0x]

I should look at that Python fail later.

[oir]

Looks great! Ty for this contribution. I will document the new feature and bump version / release soon.