feature #384: Specify custom map key sort function

[zachelrath]

Addresses #384 , adding the ability to specify a custom map key sort function via a registered extension. Defaults to the previous behavior, of just sorting map keys alphanumerically using < operator

Example use:

type prioritizedSorter struct {
   prioritizedKey string
}

func (sorter *prioritizedSorter) Sort(keyA string, keyB string) bool {
	// Prioritize our configured key, otherwise alpha-sort
	if keyA == prioritizedKey {
		return true
	} else if keyB == prioritizedKey {
		return false
	} else {
		return keyA < keyB
	}
}

type prioritizedSortExtension struct {
   jsoniter.DummyExtension
   sorter prioritizedSorter 
}

func (extension *prioritizedSortExtension) CreateMapKeySorter() jsoniter.MapKeySorter {
	return extension.sorter
}

...

cfg := jsoniter.Config{
   SortMapKeys: true,
}.Froze()

cfg.RegisterExtension(&prioritizedSortExtension{
   sorter: &prioritizedSorter{
      prioritizedKey: "trump",
   },
})

m := map[string]int{
   "b":  "sdfasdf"
    "trump": "i should be first",
   "a":  "asdfasf",
}
output, _ := cfg.MarshalToString(m)

[codecov]

Codecov Report

Merging #385 into master will increase coverage by 0.12%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #385      +/-   ##
==========================================
+ Coverage   81.64%   81.76%   +0.12%     
==========================================
  Files          41       41              
  Lines        5022     5041      +19     
==========================================
+ Hits         4100     4122      +22     
+ Misses        801      798       -3     
  Partials      121      121

Impacted Files	Coverage Δ
reflect.go	92.7% <ø> (ø)	⬆️
reflect_map.go	88.26% <100%> (+0.76%)	⬆️
reflect_extension.go	83.4% <33.33%> (+0.41%)	⬆️
reflect_struct_decoder.go	48.32% <0%> (+0.41%)	⬆️

---

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