Added svg defs to be used as fills for individual days#81

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daxaxelrod wants to merge 2 commits intokevinsqi:masterfrom

daxaxelrod:svg_gradients

Commits on Jul 20, 2018

Added svg defs to be used as fills for individual days. Built upon the value cache used for classes
daxaxelrod
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fixed the gradient fill prop reference, added proptypes entry for defs and description
daxaxelrod
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