No Support for Multiple Line Chart CircleHole colors

* [x] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## Feature Request

I tried to update the circleHoles of a Line Chart to have different colors similarly to how the actual circles can have different colors. However, it seems as if circleHole color can only be defined on a per data set basis.

## Charts Environment

**Charts version/Branch/Commit Number:**
v3.2.1 master branch
**Xcode version:**
Xcode 10.0
**Swift version:**
Swift 4.2

## Demo Project
The current ChartsDemo-iOS-Swift workspace

Start by reproducing the request in the current ChartsDemo-iOS-Swift workspace with Swift 4.2 and Charts v3.2.1, then trace how Line Chart circleHole colors are currently configured per data set. Done means a line chart can assign different circleHole colors while preserving existing behavior and the demo or relevant tests show the result.

Written by the indexing model from the issue text.