text: Title: Flexible options for color palette.
  Body: Currently, the color palette is hardcoded in visuals.py:
  https://github.com/MLWave/kepler-mapper/blob/0dfa7f736bd4218e8996888785b9d2adc63d5344/kmapper/visuals.py#L8-L15
  
  and again in kmapper.js
  
  https://github.com/MLWave/kepler-mapper/blob/0dfa7f736bd4218e8996888785b9d2adc63d5344/kmapper/static/kmapper.js#L25-L32
  
  I'd like to see this 
  - [ ] duplication fixed and 
  - [ ] allow the palette to be an argument so users can choose a custom palette.
  
  Reducing the duplication should be straight forward. Allowing arbitrary palettes might be more difficult.  Does the current code assume palettes of a certain length?