NM Various Enhancements

[neomantra]

Description

Please share your ideas and awesome changes.

I've been loving this library -- such a great integration of Golang <> JSON <> eCharts. It took me a while to isolate my changes, but I finally rebased them for a PR. Since one has a breaking change, maybe you want to separate it and I will if you ask me to.

Three changes, one adds some missing fields:

   * Tooltip.ValueFormatter
   * AxisPointer.Show
   * AxisPointer.Label
   * AxisLabel.BackgroundColor
   * XAxis.AxisPointer
   * YAxis.AxisPointer
   * DataZoom.LabelFormatter

Another strips // comments from JavaScript code, while still having FuncNoStripOpts if somehow that's not wanted. But most JavaScript code will. This makes using go:embed much easier (which I'll add a PR to examples about).

The last adds support for multiple datasets. This is great for issues like It has #169 with multiple charts. This has one breaking change, but in my opinion it adds so much power and harmonizes the syntax:

    // old
    chart.Dataset = opts.Dataset{Source: test}
    // new
    chart.AddDataset(opts.Dataset{Source: test})

---

Type of change

• Bug fix (Non-breaking change which fixes an issue)
• New feature (Non-breaking change which adds functionality)
• Breaking change (Would cause existing functionality to not work as expected)
• Others

---

Examples:

• Yes, I am willing to submit a PR on Examples!

A multi-dataset example would be super useful. I don't have much time but hope to get to it.

I also want to make an example using go:embed since it is so powerful (also needs go 1.16 whereas examples only uses go 1.15). But I won't have time for a while too. Here's the idea:

// go:embed js/tooltip.js
var tooltipFormatter string
....
charts.WithTooltipOpts(opts.Tooltip{
	Show:    true,
	Trigger: "axis",
	Formatter: opts.FuncOpts(tooltipFormatter),
}

js/tooltip.js

function (params) {
    // here's a comment!
    console.log(params);
}

[Koooooo-7]

LGTM.