Added ColorPropType

Summary:
Problem: https://github.com/facebook/react-native/issues/4708

Solution: Added a ColorPropType that validates the color used by the dev

Notes:
1) I'm working a Win8.1 machine and couldn't build the react-native using the github repo. As soon as I figure that out, I'll probably figure how to run the tests and how to add some for this feature.
2) It's my first pull request. Be gentle :)
Closes https://github.com/facebook/react-native/pull/4866

Reviewed By: bestander, svcscm

Differential Revision: D2783672

Pulled By: nicklockwood

fb-gh-sync-id: ca22aa3c0999188075681b5d20fff0631496e238

Libraries/Text/TextStylePropTypes.js

@@ -12,11 +12,12 @@
 'use strict';
 
 var ReactPropTypes = require('ReactPropTypes');
+var ColorPropType = require('ColorPropType');
 var ViewStylePropTypes = require('ViewStylePropTypes');
 
 // TODO: use spread instead of Object.assign/create after #6560135 is fixed
 var TextStylePropTypes = Object.assign(Object.create(ViewStylePropTypes), {
-  color: ReactPropTypes.string,
+  color: ColorPropType,
   fontFamily: ReactPropTypes.string,
   fontSize: ReactPropTypes.number,
   fontStyle: ReactPropTypes.oneOf(['normal', 'italic']),
@@ -55,7 +56,7 @@ var TextStylePropTypes = Object.assign(Object.create(ViewStylePropTypes), {
   /**
    * @platform ios
    */
-  textDecorationColor: ReactPropTypes.string,
+  textDecorationColor: ColorPropType,
   /**
    * @platform ios
    */