Feature: scaleType: diverging

[clhenrick]

Add support for diverging scales.

It might work like:

<color-legend
  scaleType="diverging"
  domain=[0, 0.5, 1]
  range=['red', 'yellow', 'blue']
/>

[clhenrick]

Good discussion on diverging color scales, in particular whether or not it's okay to apply equal luminosity to values on either end of the range: https://twitter.com/inari_ta/status/1511271844564897796