Implement support for Consent strings v2 (#2)

Co-authored-by: Luis Vieira <[email protected]>

Author: Florian Hartwig

README.md

@@ -6,7 +6,9 @@ A minimalistic Go library to encode and decode [IAB consent][iab] strings.
 
 [iab]: https://github.com/InteractiveAdvertisingBureau/GDPR-Transparency-and-Consent-Framework/blob/68f5e0012a7bdb00867ce9fee57fb67cfe9153e3/Consent%20string%20and%20vendor%20list%20formats%20v1.1%20Final.md
 
-### Usage example
+### Usage examples
+
+#### Version 1
 
 ```go
 package main
@@ -48,3 +50,34 @@ func main() {
 	fmt.Println(c2.String())
 }
 ```
+
+#### Version 2
+
+```go
+	// Decode a consent v2 string
+	cv2, err := consent.ParseV2("COtybn4PA_zT4KjACBENAPCIAEBAAECAAIAAAAAAAAAA")
+
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+	fmt.Printf("Last modified: %s, vendors allowed: %v\n",
+		cv2.LastUpdated, cv2.VendorConsent)
+```
+
+#### Consent string of unknown version
+
+```go
+	// decode a consent string without knowing the version beforehand
+	cvx, err := consent.Parse("BOQ7WlgOQ7WlgABACDENABwAAABJOACgACAAQABA")
+	if err != nil {
+	fmt.Println(err)
+		os.Exit(1)
+	}
+	switch c3 := cvx.(type) {
+	case *consent.ConsentV1:
+		fmt.Printf("V1. PurposesAllowed: %v\n", c3.PurposesAllowed)
+	case *consent.ConsentV2:
+		fmt.Printf("V2. PurposesConsent: %v\n", c3.PurposesConsent)
+	}
+```

bits.go

@@ -29,6 +29,14 @@ func (b *bitWriter) AppendBools(bools []bool) {
 	}
 }
 
+func (b *bitWriter) AppendBit(x bool) {
+	var v byte
+	if x {
+		v = 1
+	}
+	b.AppendByte(v, 1)
+}
+
 func (b *bitWriter) AppendBits(bits bitWriter) {
 	for i := range bits.data {
 		b.AppendByte(bits.data[i], 8)
@@ -106,3 +114,8 @@ func (b *bitReader) ReadInt(size uint) (int64, bool) {
 	}
 	return out, true
 }
+
+func (b *bitReader) ReadBit() (bool, bool) {
+	v, ok := b.ReadByte(1)
+	return v > 0, ok
+}