feat(render): implement rendering pieces as a render-prop

[csharptest]

provides the ability to pass a render function as a pieces render property, the single argument provides width, height, and piece.

example:

<Chessboard
  pieces={{
    wQ: ({width, height, piece}) => (<div>queen</div>)
  }}
/>

[codecov-io]

Codecov Report

Merging #20 into master will decrease coverage by 0.11%. The diff coverage is 50%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
- Coverage   65.28%   65.16%   -0.12%     
==========================================
  Files          11       11              
  Lines         265      267       +2     
  Branches       62       63       +1     
==========================================
+ Hits          173      174       +1     
- Misses         71       72       +1     
  Partials       21       21

Impacted Files	Coverage Δ
src/Chessboard/RenderPieces.js	76.19% <50%> (-2.76%)	:arrow_down:

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[willb335]

I like this feature. This should clean up quite a bit of code as well. I have some time tomorrow to take a look at it. Thanks!

[willb335]

Moved to #22