Add rainbow styling

![reading-rainbow](https://f.cloud.github.com/assets/1027776/1063261/674fc10c-129d-11e3-81a0-4a5a96d55462.gif)

Color.js supports a feature that outputs a rainbow colored string.  My Yeoman generator used it and I am working on converting it over to chalk.  I can change the display, especially since rainbow text is kind of like this:
![puking-rainbow](https://f.cloud.github.com/assets/1027776/1063265/951e1eee-129d-11e3-8934-bb8ceea70211.gif)

But, if you would like to support it, here is the feature...