Support token offsets in token writer

Change-Id: I7db20d8f26fd87a2f43c3a6599bfeba050fc340a
diff --git a/fomafile.go b/fomafile.go
index 2b10736..33128a2 100644
--- a/fomafile.go
+++ b/fomafile.go
@@ -29,6 +29,7 @@
 
 type Tokenizer interface {
 	Transduce(r io.Reader, w io.Writer) bool
+	TransduceTokenWriter(r io.Reader, w *TokenWriter) bool
 	Type() string
 }