Add support to time scale with colorStop option configured with formatted dates

[stockiNail]

Fix #15.

This PR is enable the feature to use in color node color stop as formatted date, leveraging on scale parsing.

Example:

gradient: {
  backgroundColor: {
    axis: 'x',
      colors: {
        "05/30/2021": 'green',
        "06/02/2021": 'blue',
        "06/04/2021": 'red'
      }
    }
  }
}

Time scale options:

scales: {
  x: {
    type: 'time', 
    time: {
      unit: 'day',
      parser: 'MM/dd/yyyy',
    }
  }
}

[stockiNail]

let me rebase

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[stockiNail]

This PR is also enabling the colorStop at relative index on labels for category axes.

Instead of using the label, you can use the index of the label adding adjustment, like:

gradient: {
  backgroundColor: {
    axis: 'x',
      colors: {
        "0": 'green', // position of label at index 0
        "2.5": 'blue', // position of the label between labels at index 2 and 3
        "8.5": 'red' // position of the label between labels at index 8 and 9
      }
    }
  }
}

[stockiNail]

@kurkle thank you very much for quick approval and merge. Now I can start working to import the plugin in my lib. ;)

[kurkle]

I'll delay the release though, because I'd like to add some docs and samples before that. Maybe call it 1.0 too.

[stockiNail]

I'll delay the release though,

Take your time. Because I have to develop the java wrappers and do all tests, I'm going to starting working with my fork and the file created locally. I'll change when new version will be published.

because I'd like to add some docs and samples before that.

Even if I'm busy (but luckily I can still work in multi threading...) maybe I can help you somehow. Let me know.

Maybe call it 1.0 too.

Vote for it!