feat(TextArea): add focus() method

[gnowoel]

I want to focus a TextArea element with a ref, but the ref refers to a component instance rather than a DOM element. We already solved this problem for Input by adding a focus() method, which will relay the method invocation to the underlying DOM element. So, in this PR, I'd like to add the same focus() method to the TextArea component.

[codecov-io]

Codecov Report

Merging #1602 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1602      +/-   ##
==========================================
+ Coverage   99.74%   99.74%   +<.01%     
==========================================
  Files         141      141              
  Lines        2396     2397       +1     
==========================================
+ Hits         2390     2391       +1     
  Misses          6        6

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

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[layershifter]

@gnowoel Thanks for PR, I've updated a test, now lint passes. LGTM 👍

[levithomason]

Thanks much guys!

[gnowoel]

@layershifter Thanks much for the test update! I felt frustrated about the failed tests since they passed on localhost :)

[levithomason]

Released in semantic-ui-react@0.68.1.