Implementing localization

[Frondor]

First try, WIP, waiting for reviews

Besides of implementing localization, this PR adds a new way to work with the library with more flexibility without breaking changes.

A full example of the new APIs and method features

// optionally, you can use a named import to get the Dayjs class and pass your own configuration object.
import { Dayjs }, dayjs from 'dayjs'

// Instantiate your own Dayjs object, and use it anywhere in your app with the provided default config
// myDayjs.js
import { Dayjs }, dayjs from 'dayjs'
import ESP from 'dayjs/locales/es'

// You can still do
dayjs().format() // "2018-04-01T23:10:59Z"

// all of these settings are optional
const config = {
    date: new Date(), // optional
    locale: ESP, // default locale for this and cloned instances
    format: 'YYYY / MM / DD HH:ss', // default format, used for empty calls to format() and object serialization
    // more config...
}

export default new Dayjs(config)

/*
* ----------------------------------
*/
// Somewhere in your app
import myDayjs from '../myDayjs'

myDayjs.set('year', 1991).format() // "1991 / 12 / 1 09:30"
myDayjs.format('YYYY') // "2018"
myDayjs
    .set(new Date(2018, 5, 1)) // you can now pass a Date or Dayjs instance to set!
    .setLocale() // change the instance locale from spanish back to english
    .format('dddd - MMMM', {
        // override default format and WeekDays locale (only WD)
        WD: ['Mondy', 'Tuesdy', 'Wendy', 'Thursdy', 'Fridy', 'Saty', 'Sundy']
    }) // "Tuesdy - May"

// And now, objects with Dayjs instances are being serialized to a JSON string using the default format
JSON.stringify({ name: 'Jon', birthday: myDayjs }) //  '{name: "Jon", birthday: "2018 / 12 / 1 09:30"}'

closes #36

[codecov-io]

Codecov Report

Merging #77 into dev will not change coverage.
The diff coverage is 100%.

@@         Coverage Diff         @@
##            dev    #77   +/-   ##
===================================
  Coverage   100%   100%           
===================================
  Files         3      3           
  Lines       175    188   +13     
  Branches     27     34    +7     
===================================
+ Hits        175    188   +13

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

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[atilacamurca]

I think this could be improved by using:

format(formatStr = this.$format, L = this.$L)

Then I think you can get rid of next lines short circuits ||. What do you think?

[Frondor]

Really depends, because you may do format.('...', { WD: [0,1,2,3,4,5,6] }) to override the WeekDays locale, but leave MONTHS untouched with the instance's default locale. You're providing overrides for certain locales, not the whole localized object.

[Frondor]

Switching target branch to @next

[iamkun]

@Frondor

myDayjs.set(new Date(2018, 5, 1)) // you can now pass a Date or Dayjs instance to set!

Could you explain the meaning of you code here, please? Don't really understand.

[Frondor]

Sure, this code is kind of explained in the test case https://github.com/Frondor/dayjs/blob/5f574850d1cb75c6bd2a8b1684a94346f1d501b7/test/get-set.test.js#L69
In the past, you were limited to set certain time units over an already defined date ($d). Now that code checks whether the unitOrDate passed is either a time unit, a Date object, or a Dayjs instance. So you can set the whole date if needed, not just an unit. Useful for inheriting configurations like locale, plugins and default format, and create date objects out of a single source, and keeping the Dayjs immutable since that's how set works.
By the way, just for the record, this is the old PR, the new one is #90

[Frondor]

set(unitOrDate, int) {
    // if unitOrDate is a Date or Dayjs object
    if (typeof unitOrDate === 'object' &&
      unitOrDate.constructor.name.indexOf(['Date', 'Dayjs'] > -1)) {
      const self = this.clone() // don't mutate this instance
     // set $d date object depending on whether its a native Date object, or a Dayjs instance
      self.$d = unitOrDate.toDate ? unitOrDate.toDate() : unitOrDate
      return self // done
    }
    // if this is not a date object nor an int just fail silently (this code was already here)
    if (!Utils.isNumber(int)) return this
   // unitOrDate is definitely a integer (unit)
    return this.clone().mSet(unitOrDate, int)
  }

[iamkun]

@Frondor Got it, thanks.