encoding/xml: add (*Encoder).Indent

Exposing this on the Encoder allows streaming generation of indented XML.

R=golang-dev, rogpeppe
CC=golang-dev
https://golang.org/cl/7221075

src/pkg/encoding/xml/marshal.go

@@ -81,8 +81,7 @@ func Marshal(v interface{}) ([]byte, error) {
 func MarshalIndent(v interface{}, prefix, indent string) ([]byte, error) {
 	var b bytes.Buffer
 	enc := NewEncoder(&b)
-	enc.prefix = prefix
-	enc.indent = indent
+	enc.Indent(prefix, indent)
 	if err := enc.Encode(v); err != nil {
 		return nil, err
 	}
@@ -99,6 +98,14 @@ func NewEncoder(w io.Writer) *Encoder {
 	return &Encoder{printer{Writer: bufio.NewWriter(w)}}
 }
 
+// Indent sets the encoder to generate XML in which each element
+// begins on a new indented line that starts with prefix and is followed by
+// one or more copies of indent according to the nesting depth.
+func (enc *Encoder) Indent(prefix, indent string) {
+	enc.prefix = prefix
+	enc.indent = indent
+}
+
 // Encode writes the XML encoding of v to the stream.
 //
 // See the documentation for Marshal for details about the conversion