implement format strings base64, base64d (ref: #2)

cli/test.yaml

@@ -3211,6 +3211,22 @@
   expected: |
     "{"foo":"\\u003cdiv\\u003e\\u0026'\\"\\u003c/div\\u003e"}"
 
+- name: format strings @base64
+  args:
+    - '@base64'
+  input: |
+    {"foo": "bar"}
+  expected: |
+    "eyJmb28iOiJiYXIifQ=="
+
+- name: format strings @base64d
+  args:
+    - -R
+    - '@base64d'
+  input: eyJmb28iOiJiYXIifQ==
+  expected: |
+    "{\"foo\":\"bar\"}"
+
 - name: destructuring alternative operator
   args:
     - '.[] | . as {a:$a} ?// [$a] ?// $a | $a'

compiler.go

@@ -910,6 +910,10 @@ func (c *compiler) compileFormat(fmt string) error {
 		return c.compileFunc(&Func{Name: "tojson"})
 	case "@html":
 		return c.compileFunc(&Func{Name: "_tohtml"})
+	case "@base64":
+		return c.compileFunc(&Func{Name: "_tobase64"})
+	case "@base64d":
+		return c.compileFunc(&Func{Name: "_tobase64d"})
 	default:
 		return &formatNotFoundError{fmt}
 	}

func.go

@@ -1,6 +1,7 @@
 package gojq
 
 import (
+	"encoding/base64"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -58,6 +59,8 @@ func init() {
 		"tojson":         argFunc0(funcToJSON),
 		"fromjson":       argFunc0(funcFromJSON),
 		"_tohtml":        argFunc0(funcToHTML),
+		"_tobase64":      argFunc0(funcToBase64),
+		"_tobase64d":     argFunc0(funcToBase64d),
 		"_index":         argFunc2(funcIndex),
 		"_slice":         argFunc3(funcSlice),
 		"_break":         argFunc0(funcBreak),
@@ -499,6 +502,28 @@ func funcToHTML(v interface{}) interface{} {
 	}
 }
 
+func funcToBase64(v interface{}) interface{} {
+	switch x := funcToString(v).(type) {
+	case string:
+		return base64.StdEncoding.EncodeToString([]byte(x))
+	default:
+		return x
+	}
+}
+
+func funcToBase64d(v interface{}) interface{} {
+	switch x := funcToString(v).(type) {
+	case string:
+		y, err := base64.StdEncoding.DecodeString(x)
+		if err != nil {
+			return err
+		}
+		return string(y)
+	default:
+		return x
+	}
+}
+
 func funcIndex(_, v, x interface{}) interface{} {
 	switch x := x.(type) {
 	case string: