Update colors

[chanind]

Closes #87

This PR adds an updateColor(colorName, newValue, options) method which can be used to tween colors on a running HanziWriter instance.

[codecov-io]

Codecov Report

Merging #88 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
+ Coverage   98.41%   98.46%   +0.05%     
==========================================
  Files          22       22              
  Lines         885      915      +30     
  Branches      147      157      +10     
==========================================
+ Hits          871      901      +30     
  Misses         14       14

Impacted Files	Coverage Δ
src/characterActions.js	100% <100%> (ø)	⬆️
src/utils.js	100% <100%> (ø)	⬆️
src/renderers/UserStrokeRenderer.js	100% <100%> (ø)	⬆️
src/renderers/CharacterRenderer.js	100% <100%> (ø)	⬆️
src/HanziWriter.js	93.47% <100%> (+0.45%)	⬆️
src/RenderState.js	100% <100%> (ø)	⬆️
src/renderers/StrokeRenderer.js	100% <100%> (ø)	⬆️
src/renderers/HanziWriterRenderer.js	100% <100%> (ø)	⬆️

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