Add rebeccapurple to fit Named Colors of CSS Color Module Level 4

[CSS Color Module Level 4](https://drafts.csswg.org/css-color/#named-colors)

Author: toptalo

src/css/Colors.js

@@ -123,6 +123,7 @@ var Colors = module.exports = {
     plum                    : "#dda0dd",
     powderblue              : "#b0e0e6",
     purple                  : "#800080",
+    rebeccapurple           : "#663399",
     red                     : "#ff0000",
     rosybrown               : "#bc8f8f",
     royalblue               : "#4169e1",