[FEATURE] Add tooltip textLabelColor callback

[Elolawyn]

Expected Behavior

I want to be able to apply data color (bar color) to tooltip label:

tooltips: {
  callbacks: {
    textLabelColor: (tooltipItem, chartInstance) => {
      return chartInstance.data.datasets[tooltipItem.datasetIndex].backgroundColor;
    }
  }
}

Current Behavior

There is a callback called labelColor that can be used to set the color of the box that appears right before the label but not the text.

[etimberg]

Happy to look at a PR for this.