Chain transformers and test cases

Transformers can now be chained together, working on the output of the
previous run.

transform/chain.go

@@ -0,0 +1,29 @@
+package transform
+
+import (
+	"io"
+	"bytes"
+)
+
+type chain struct {
+	transformers []Transformer
+}
+
+func NewChain(trs ...Transformer) Transformer {
+	return &chain{transformers: trs}
+}
+
+func (c *chain) Apply(r io.Reader, w io.Writer) (err error) {
+	in := r
+	for _, tr := range c.transformers {
+		out := new(bytes.Buffer)
+		err = tr.Apply(in, out)
+		if err != nil {
+			return
+		}
+		in = bytes.NewBuffer(out.Bytes())
+	}
+	
+	_, err = io.Copy(w, in)
+	return
+}