Make any cursor colors visible

[ohtake]

#12 has a font-related issue in different browsers.
This pull request extract commit relating to cursor colors from #12.

[coveralls]

Coverage remained the same at 80.473% when pulling e6fbd4b on cursor-color into 9b09351 on master.

[codecov-io]

Codecov Report

Merging #13 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #13   +/-   ##
=======================================
  Coverage   79.42%   79.42%           
=======================================
  Files          19       19           
  Lines         904      904           
  Branches      106      106           
=======================================
  Hits          718      718           
  Misses        152      152           
  Partials       34       34

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