Add rebeccapurple

http://dev.w3.org/csswg/css-color/#valdef-color-rebeccapurple

Btw, please do not release something under 0.0.x Nobody can get fixes this way :(

Please release at least 0.1.0. 1.0.0 is even better since the format of the json is not likely to change soon am I correct?

Why: to fix the bug introduced in [color 0.7.3](https://github.com/harthur/color-string/commit/426e35416c6c776443898eb60b8043e1044ebc8f) we will need to update color-string again.