feat(Embed): add iframe shorthand

[layershifter]

Fixes #1804.

[layershifter]

@levithomason There is single failing test:

iframe shorthand prop (common)
✖ renders a iframe from elements

I can't fix it, looks like a problem with default value src, but nothing changes if I define it in shorthandDefaultProps. BTW, we can simply pick the strictAssert feature from #1795.

[layershifter]

Requires #1845.

[codecov-io]

Codecov Report

Merging #1810 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1810      +/-   ##
==========================================
+ Coverage   99.75%   99.75%   +<.01%     
==========================================
  Files         145      145              
  Lines        2477     2478       +1     
==========================================
+ Hits         2471     2472       +1     
  Misses          6        6

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

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[layershifter]

Unblocked, ready for review 👍

[levithomason]

👍

[levithomason]

Released in semantic-ui-react@0.71.3