Add helper to print message with hash as a header.

Broke out email.go's Hash() in to multiple functions to make this easier, and
update tools that used it.

cmd/addhashheader/addhashheader.go

@@ -0,0 +1,36 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"net/mail"
+	"os"
+
+	"github.com/golang/glog"
+
+	"xinu.tv/email"
+)
+
+func main() {
+	flag.Parse()
+	defer glog.Flush()
+
+	msg, err := mail.ReadMessage(os.Stdin)
+	if err != nil {
+		glog.Fatal(err)
+	}
+
+	h, err := email.HashMessage(msg)
+	if err != nil {
+		glog.Fatal(err)
+	}
+	for k, vs := range msg.Header {
+		for _, v := range vs {
+			fmt.Printf("%s: %s\n", k, v)
+		}
+	}
+	fmt.Println("X-Xinu-Hash:", h)
+	fmt.Println()
+	io.Copy(os.Stdout, msg.Body)
+}