Write to rotating ContentReWriter in transformer chain

This commit adds the interface ContentReWriter in the tranformer chain.

This is backed by two pooled byte buffers, alternating between being the reader or the writer.

This keeps the performance characteristic of the old implementation, but in a thread safe way.

Fixes #911

Benchmark old vs new:

benchmark              old ns/op     new ns/op     delta
BenchmarkAbsURL        17614         17384         -1.31%
BenchmarkXMLAbsURL     9431          9248          -1.94%

benchmark              old allocs     new allocs     delta
BenchmarkAbsURL        24             28             +16.67%
BenchmarkXMLAbsURL     12             14             +16.67%

benchmark              old bytes     new bytes     delta
BenchmarkAbsURL        3295          3424          +3.92%
BenchmarkXMLAbsURL     1954          1987          +1.69%

transform/chain_test.go

@@ -2,6 +2,7 @@ package transform
 
 import (
 	"bytes"
+	"github.com/spf13/hugo/helpers"
 	"strings"
 	"testing"
 )
@@ -54,6 +55,35 @@ func TestChainZeroTransformers(t *testing.T) {
 	}
 }
 
+func TestChaingMultipleTransformers(t *testing.T) {
+	f1 := func(rw ContentReWriter) {
+		rw.Write(bytes.Replace(rw.Content(), []byte("f1"), []byte("f1r"), -1))
+	}
+	f2 := func(rw ContentReWriter) {
+		rw.Write(bytes.Replace(rw.Content(), []byte("f2"), []byte("f2r"), -1))
+	}
+	f3 := func(rw ContentReWriter) {
+		rw.Write(bytes.Replace(rw.Content(), []byte("f3"), []byte("f3r"), -1))
+	}
+
+	f4 := func(rw ContentReWriter) {
+		rw.Write(bytes.Replace(rw.Content(), []byte("f4"), []byte("f4r"), -1))
+	}
+
+	tr := NewChain(f1, f2, f3, f4)
+
+	out := new(bytes.Buffer)
+	if err := tr.Apply(out, helpers.StringToReader("Test: f4 f3 f1 f2 f1 The End.")); err != nil {
+		t.Errorf("Multi transformer chain returned an error: %s", err)
+	}
+
+	expected := "Test: f4r f3r f1r f2r f1r The End."
+
+	if string(out.Bytes()) != expected {
+		t.Errorf("Expected %s got %s", expected, string(out.Bytes()))
+	}
+}
+
 func BenchmarkAbsURL(b *testing.B) {
 	absURL, _ := AbsURL("http://base")
 	tr := NewChain(absURL...)