feature: add expandIcon

[kagd]

This has the same changes as #187, but also includes the unit tests and examples.

This PR allows for overriding the default expand icon.

[codecov-io]

Codecov Report

Merging #236 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #236      +/-   ##
=========================================
+ Coverage   93.38%   93.4%   +0.01%     
=========================================
  Files          17      17              
  Lines         726     728       +2     
  Branches      208     209       +1     
=========================================
+ Hits          678     680       +2     
  Misses         40      40              
  Partials        8       8

Impacted Files	Coverage Δ
src/ExpandableTable.js	98.21% <ø> (ø)	⬆️
src/ExpandableRow.js	97.56% <100%> (+0.12%)	⬆️

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[yesmeck]

Close ant-design/ant-design#11802

[yesmeck]

Close #186

[sytolk]

I have tested to change the default icon with this. But for the same data (with and without children properties) the results are different.
If I have expandIcon properties (the same from example) its have icon displayed on every node (without children properties).
If we have data with children: [ ] properties the result are displayed correctly only with default [+/-] icon.