feat(Ref): add component

[layershifter]

Splitted from #1879.

Why?

ref isn't a prop, it has a same behaviour as key - it never passed to a component. However, there were numerous issues about refs (#1740, #1602, #1849, #1732 and more) and it's time to make something in this way. document.querySelector is a workaround, but it's not a solution.

Problem is a well known in React's world, there is a long-live issue facebook/react#4213, there was also the RFC facebook/react#6974 from Dan.

Variants

Project	Prop	Example
formsy-react-components	componentRef & innerRef	refs.md
office-ui-fabric-react	componentRef	office-ui-fabric-react#1356
react-redux	connect HOC, withRef	reduxjs/react-redux@2d3d0be
react-router	innerRef	Link.md

This PR only adds Ref addon, all other work will be done in #1879.

[codecov-io]

Codecov Report

Merging #2142 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2142      +/-   ##
==========================================
+ Coverage   99.76%   99.76%   +<.01%     
==========================================
  Files         150      151       +1     
  Lines        2597     2601       +4     
==========================================
+ Hits         2591     2595       +4     
  Misses          6        6

Impacted Files	Coverage Δ
src/addons/Ref/Ref.js	100% <100%> (ø)

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[levithomason]

🎉

[levithomason]

Released in semantic-ui-react@0.75.0