Add @dig modifier for recursive descent searches

This commit adds the "@dig" modifier, which allows for searching
for values in deep or arbitrarily nested json documents

For example, using the following json:

```
{ "something": {
    "anything": {
      "abcdefg": {
          "finally": {
            "important": {
                "secret": "password"
            }
        }
      }
    }
  }
}
```

```
@dig:secret  ->  ["password"]
```

See #130

gjson.go

@@ -2754,6 +2754,7 @@ func execModifier(json, path string) (pathOut, res string, ok bool) {
 			var parsedArgs bool
 			switch pathOut[0] {
 			case '{', '[', '"':
+				// json arg
 				res := Parse(pathOut)
 				if res.Exists() {
 					args = squash(pathOut)
@@ -2762,14 +2763,20 @@ func execModifier(json, path string) (pathOut, res string, ok bool) {
 				}
 			}
 			if !parsedArgs {
-				idx := strings.IndexByte(pathOut, '|')
-				if idx == -1 {
-					args = pathOut
-					pathOut = ""
-				} else {
-					args = pathOut[:idx]
-					pathOut = pathOut[idx:]
+				// simple arg
+				i := 0
+				for ; i < len(pathOut); i++ {
+					if pathOut[i] == '|' {
+						break
+					}
+					switch pathOut[i] {
+					case '{', '[', '"', '(':
+						s := squash(pathOut[i:])
+						i += len(s) - 1
+					}
 				}
+				args = pathOut[:i]
+				pathOut = pathOut[i:]
 			}
 		}
 		return pathOut, fn(json, args), true
@@ -2789,19 +2796,24 @@ func unwrap(json string) string {
 // DisableModifiers will disable the modifier syntax
 var DisableModifiers = false
 
-var modifiers = map[string]func(json, arg string) string{
-	"pretty":  modPretty,
-	"ugly":    modUgly,
-	"reverse": modReverse,
-	"this":    modThis,
-	"flatten": modFlatten,
-	"join":    modJoin,
-	"valid":   modValid,
-	"keys":    modKeys,
-	"values":  modValues,
-	"tostr":   modToStr,
-	"fromstr": modFromStr,
-	"group":   modGroup,
+var modifiers map[string]func(json, arg string) string
+
+func init() {
+	modifiers = map[string]func(json, arg string) string{
+		"pretty":  modPretty,
+		"ugly":    modUgly,
+		"reverse": modReverse,
+		"this":    modThis,
+		"flatten": modFlatten,
+		"join":    modJoin,
+		"valid":   modValid,
+		"keys":    modKeys,
+		"values":  modValues,
+		"tostr":   modToStr,
+		"fromstr": modFromStr,
+		"group":   modGroup,
+		"dig":     modDig,
+	}
 }
 
 // AddModifier binds a custom modifier command to the GJSON syntax.
@@ -3435,3 +3447,30 @@ func escapeComp(comp string) string {
 	}
 	return comp
 }
+
+func parseRecursiveDescent(all []Result, parent Result, path string) []Result {
+	if res := parent.Get(path); res.Exists() {
+		all = append(all, res)
+	}
+	if parent.IsArray() || parent.IsObject() {
+		parent.ForEach(func(_, val Result) bool {
+			all = parseRecursiveDescent(all, val, path)
+			return true
+		})
+	}
+	return all
+}
+
+func modDig(json, arg string) string {
+	all := parseRecursiveDescent(nil, Parse(json), arg)
+	var out []byte
+	out = append(out, '[')
+	for i, res := range all {
+		if i > 0 {
+			out = append(out, ',')
+		}
+		out = append(out, res.Raw...)
+	}
+	out = append(out, ']')
+	return string(out)
+}

gjson_test.go

@@ -2636,3 +2636,68 @@ func TestIssue301(t *testing.T) {
 	assert(t, Get(json, `fav\.movie.[1]`).String() == "[]")
 
 }
+
+func TestModDig(t *testing.T) {
+	json := `
+		{
+
+			"group": {
+				"issues": [
+					{
+						"fields": {
+						"labels": [
+							"milestone_1",
+							"group:foo",
+							"plan:a",
+							"plan:b"
+						]
+						},
+						"refid": "123"
+					},{
+						"fields": {
+						"labels": [
+							"milestone_2",
+							"group:foo",
+							"plan:a",
+							"plan"
+						]
+						},
+						"refid": "456"
+					},[
+						{"extra_deep":[{
+							"fields": {
+							"labels": [
+								"milestone_3",
+								"group:foo",
+								"plan:a",
+								"plan"
+							]
+							},
+							"refid": "789"
+						}]
+					}]
+				]
+			}
+		}
+	`
+	assert(t, Get(json, "group.@dig:#(refid=123)|0.fields.labels.0").String() == "milestone_1")
+	assert(t, Get(json, "group.@dig:#(refid=456)|0.fields.labels.0").String() == "milestone_2")
+	assert(t, Get(json, "group.@dig:#(refid=789)|0.fields.labels.0").String() == "milestone_3")
+	json = `
+	{ "something": {
+		"anything": {
+		  "abcdefg": {
+			  "finally": {
+				"important": {
+					"secret": "password",
+					"name": "jake"
+				}
+			},
+			"name": "melinda"
+		  }
+		}
+	  }
+	}`
+	assert(t, Get(json, "@dig:name").String() == `["melinda","jake"]`)
+	assert(t, Get(json, "@dig:secret").String() == `["password"]`)
+}