Use a date selector for slider input fields with a `Datetime` axis

[AleksueiR]

Read #4 (comment) for more information.

[AleksueiR]

Okay, this is way more complicated than I expected. Time is hard.

NoUISlider library doesn't handle dates directly, but it work with Linux timestamps. The Highcharts also accepts Linux timestamps. To properly use dates, chart data should already contain proper date values in the form of timestamps or other formats that Highcharts can interpret as dates (YYYY-mm-dd for example, etc.). If the data does not have timestamps and values on the X-axis are generated with a simple interval, the start point, interval and interval unit need to be specified. Something like this:

plotOptions: {
                            area: {
                                pointStart: Date.UTC(1940, 0),
                                pointInterval: 1,
                                pointIntervalUnit: 'year',
}}

The current idea is to provide three options for the slider's input fields:

• year
• year and month
• year, month, day

The config author needs to specify the correct date option on the slider config. Otherwise, the slider will default to treating the timestamps as actual values. The slider will work, but the value will be incomprehensible to the users:

[AleksueiR]

Some useful links:
https://acdcjunior.github.io/how-bind-date-object-to-input-date-vue.js-v-model.html
https://jsfiddle.net/r3cjf4x5/45/
https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement