Open access to the underlying byte slice

This commit opens up access to the byte slice of the underlying
`bufio.Scanner`. It can be used to get access to the most recent error
when e.g. a line cannot be parsed correctly. See the code in
`ExampleReader_Bytes` for an example.

CONTRIBUTORS

@@ -1 +1,2 @@
+edhaight [@edhaight](https://github.com/edhaight)
 Manuel Laflamme [@mlaflamm](https://github.com/mlaflamm)

example_test.go

@@ -34,6 +34,27 @@ func ExampleReader() {
 	// London
 }
 
+func ExampleReader_Bytes() {
+	r := ndjson.NewReader(strings.NewReader(`{"city":"Munich"}
+{"city":"Invalid"
+{"city":"London"}`))
+	for r.Next() {
+		var loc Location
+		if err := r.Decode(&loc); err != nil {
+			fmt.Printf("Decode failed: %v. Last read: %s\n", err, string(r.Bytes()))
+			return
+		}
+		fmt.Println(loc.City)
+	}
+	if err := r.Err(); err != nil {
+		fmt.Fprintf(os.Stderr, "Reader failed: %v", err)
+		return
+	}
+	// Output:
+	// Munich
+	// Decode failed: unexpected end of JSON input. Last read: {"city":"Invalid"
+}
+
 func ExampleWriter() {
 	locations := []Location{
 		{City: "Munich"},

reader.go

@@ -52,6 +52,13 @@ func (r *Reader) Next() bool {
 	return r.s.Scan()
 }
 
+// Bytes returns the most recent buffer generated by a call to Next.
+// The underlying array may point to data that will be overwritten
+// by a subsequent call to Next. It does no allocation.
+func (r *Reader) Bytes() []byte {
+	return r.s.Bytes()
+}
+
 // Err returns the first non-EOF error that was encountered by the Reader.
 func (r *Reader) Err() error {
 	return r.s.Err()