added new duration plugin

[waseemahmad31]

Fix issue #414

Added new duration plugin

  const dayjsA = dayjs(new Date(2018, 3, 3, 4, 45, 45, 400))
  const dayjsB = dayjs(new Date(2019, 4, 15, 11, 20, 34, 700))
  const result = dayjsA.duration(dayjsB)
 console.log(result)
//output
{
    years: 1,
    months: 1,
    days: 12,
    hours: 6,
    minute: 34,
    second: 49,
    millisecond:  300
}

[codecov-io]

Codecov Report

Merging #567 into dev will not change coverage.
The diff coverage is 100%.

@@         Coverage Diff         @@
##            dev   #567   +/-   ##
===================================
  Coverage   100%   100%           
===================================
  Files       153    154    +1     
  Lines       958   1004   +46     
  Branches    128    135    +7     
===================================
+ Hits        958   1004   +46

Impacted Files	Coverage Δ
src/plugin/duration/index.js	100% <100%> (ø)
src/constant.js	100% <100%> (ø)	⬆️

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[iamkun]

Thanks.

What I'm doing now is to make a duration plugin that is fully (or at least most) compatible with moment.duration.

I'll close this PR cause this is not the plugin I'm looking for.

Cheers.

Ref: #564