The plugin system allows users to extend dayjs with additional plugins,
or change existing functionality.

The plugins work by calling the extend method:

dayjs.extend({
  n: "methodName",
  m: function(arguments) {
     //
  }
})

When overwriting a function, the original function can be referenced from
inside the plugin using:

this.getOld(pluginName)

For example:

dayjs.extend({
  n: "format",
  m: function(formatStr) {
    let oldFormat = this.getOld("format")
    let oldResults = oldFormat(formatStr)
    ...
})

This commit also introduces a plugin which extends the format options to include:
-Q: Quarter of the year
-k/kk: 1-24 hour time
-ddd: Date of the month with ordinal (1st-31st)
-S: Milliseconds

…ugin authors.

Plugins now take the Dayjs (or extended subclass) as a parameter and are responsible
for installing themselves.

The extend function is chainable, for example:
dayjs.extend(plugin1).extend(plugin2)

The extend function also now takes a second boolean parameter, which will return a new
version of dayjs. Example:

var extended = dayjs.extend(plugin1, true)
extended().plugin1()
dayjs().plugin1() //Error