Immutable support

[stefvhuynh]

shallowly converts data prop from immutable object to js, preserving all props in each data point within data.

corresponding PR in victory chart: FormidableLabs/victory-chart#542

x and y accessors and other props like animate that can take a function with a datum argument will receive a shallowly converted datum (datum will be a js plain object, but the keys will be in their original forms with a few exceptions).

i.e.

x={(datum) => {
  // datum is regular js but the keys are the original immutable objects
  return datum["myNestedStructure"].get("myNestedValue") + 10;
}}
animate={
  onEnter: {
    before: (datum) => {
      // datum is regular js
      // datum.someProp will be an immutable object if it was passed in as such
    }
  }
}

[stefvhuynh]

using reduce here because js arrays and immutable Lists both have reduce and we can then return a regular array.

[stefvhuynh]

This is more of a nice-to-have. People could just always specify an accessor function if they are passing in an immutable object.

[stefvhuynh]

these methods should work for immutable@v3.8.x (current latest version) as well as for v4.0.x (in release candidate phase) when it is released.

[boygirl]

Works well for me!