Set different text color or font for each bar in BarChartDataSet

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

## What did you do?

Implemented a stacked bar chart.

## What did you expect to happen?

Say I have two BarChartDataSets. Each holds 10 values.
So I will have 10 bars for Chart1 and 10 bars for Chart2.
I want to set different text color or font for each bar in both Chart1 and Chart2. (even if they are stacked)

## What happened instead?
I can only use one color for each BarChartDataSet.

## Charts Environment

**Charts version/Branch/Commit Number:** v.3.5.0
**Xcode version:** 11.6
**Swift version:** Swift 5
**Platform(s) running Charts:** iOS
**macOS version running Xcode:** 10.15.6