Added group modifier

The new "@group" modifier allows for grouping arrays of objects.

For example, using the "@group" modifier on the following json...

  {"id":["123","456","789"],"val":[2,1]}

will results in...

  [{"id":"123","val":2},{"id":"456","val":1},{"id":"789"}]

gjson.go

@@ -2671,6 +2671,7 @@ var modifiers = map[string]func(json, arg string) string{
 	"values":  modValues,
 	"tostr":   modToStr,
 	"fromstr": modFromStr,
+	"group":   modGroup,
 }
 
 // AddModifier binds a custom modifier command to the GJSON syntax.
@@ -2972,6 +2973,41 @@ func modToStr(str, arg string) string {
 	return string(data)
 }
 
+func modGroup(json, arg string) string {
+	res := Parse(json)
+	if !res.IsObject() {
+		return ""
+	}
+	var all [][]byte
+	res.ForEach(func(key, value Result) bool {
+		if !value.IsArray() {
+			return true
+		}
+		var idx int
+		value.ForEach(func(_, value Result) bool {
+			if idx == len(all) {
+				all = append(all, []byte{})
+			}
+			all[idx] = append(all[idx], ("," + key.Raw + ":" + value.Raw)...)
+			idx++
+			return true
+		})
+		return true
+	})
+	var data []byte
+	data = append(data, '[')
+	for i, item := range all {
+		if i > 0 {
+			data = append(data, ',')
+		}
+		data = append(data, '{')
+		data = append(data, item[1:]...)
+		data = append(data, '}')
+	}
+	data = append(data, ']')
+	return string(data)
+}
+
 // stringHeader instead of reflect.StringHeader
 type stringHeader struct {
 	data unsafe.Pointer

gjson_test.go

@@ -2467,3 +2467,39 @@ func TestToFromStr(t *testing.T) {
 	res := Get(json, "Message.@fromstr.Records.#.eventVersion.@tostr").Raw
 	assert(t, res == `["\"2.1\""]`)
 }
+
+func TestGroup(t *testing.T) {
+	json := `{"id":["123","456","789"],"val":[2,1]}`
+	res := Get(json, "@group").Raw
+	assert(t, res == `[{"id":"123","val":2},{"id":"456","val":1},{"id":"789"}]`)
+
+	json = `
+{
+	"issues": [
+	  {
+		"fields": {
+		  "labels": [
+			"milestone_1",
+			"group:foo",
+			"plan:a",
+			"plan:b"
+		  ]
+		},
+		"id": "123"
+	  },{
+		"fields": {
+		  "labels": [
+			"milestone_1",
+			"group:foo",
+			"plan:a",
+			"plan"
+		  ]
+		},
+		"id": "456"
+	  }
+	]
+  }
+  `
+	res = Get(json, `{"id":issues.#.id,"plans":issues.#.fields.labels.#(%"plan:*")#|#.#}|@group|#(plans>=2)#.id`).Raw
+	assert(t, res == `["123"]`)
+}