Screenshots for documentation
![image](https://user-images.githubusercontent.com/8811312/51517115-9f06c400-1dce-11e9-992b-14f28c3e25b9.png)
![examplecubehelix](https://user-images.githubusercontent.com/8811312/51517402-6c110000-1dcf-11e9-8b04-8e574d39d6d7.png)
Adds cubehelix palette generation.
Screenshots for documentation
![image](https://user-images.githubusercontent.com/8811312/51517115-9f06c400-1dce-11e9-992b-14f28c3e25b9.png) ![examplecubehelix](https://user-images.githubusercontent.com/8811312/51517402-6c110000-1dcf-11e9-8b04-8e574d39d6d7.png)