Add x|y|z|rDomainSort props

[mhkeller]

Per #171, this adds xDomainSort, yDomainSort, zDomainSort and rDomainSort that will kick in when the scale is ordinal.

This is a technically a breaking change because it changes the signature of the exported helper function calcUniques. . Users that aren't importing that function separately will be fine

Currently the third argument is an object with the option of { sort: true }. This PR additionally accepts per-field entries:

{  
  sort: Boolean // Overrides field-specific sort options
  x: { sort: Boolean },
  y: { sort: Boolean },
  z: { sort: Boolean },
  r: { sort: Boolean } 

The default is unsorted when using the function directly but sorts when used in your chart.

Tasks

• Implement in library
• Update tests
• Update docs

[mhkeller]

This is ready to be merged. Since it's technically a breaking change and would push it to 9.0.0, I've giving it a moment to see if anything else should go in this release.

We can preserve the old functionality and treat it as a "sort all" option and have this not be a breaking change.