feat(Label): add prompt prop

[layershifter]

This PR adds prompt prop to Label component. It used only inside Forms that can indicate errors:

Validation messages can also appear inline. UI Forms automatically format labels with the class name prompt. These validation prompts are also set to appear on input change instead of form submission.

https://semantic-ui.com/behaviors/form.html#validating-on-blur-and-other-events

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Styles applied to <Label prompt /> are different that usual:

[codecov-io]

Codecov Report

Merging #3680 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3680   +/-   ##
=======================================
  Coverage   99.83%   99.83%           
=======================================
  Files         174      174           
  Lines        3100     3100           
=======================================
  Hits         3095     3095           
  Misses          5        5

Impacted Files	Coverage Δ
src/elements/Label/Label.js	100% <100%> (ø)	⬆️

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