internal/age: remove EncryptWithArmor and armor support in Decrypt

The caller can take care of the armor. For consistency move the
responsibility to close the armor to the caller, and make the stream
Writer not propagate Close.

This also will also allow us to spin the armor implementation out into
its won package that imports format, without getting an import loop from
format.Parse magically invoking armor decoding.

Less magic in the API, more magic in the CLI.

cmd/age/age.go

@@ -7,6 +7,7 @@
 package main
 
 import (
+	"bufio"
 	"bytes"
 	"flag"
 	"fmt"
@@ -16,6 +17,7 @@ import (
 	"strings"
 
 	"filippo.io/age/internal/age"
+	"filippo.io/age/internal/format"
 	"golang.org/x/crypto/ssh/terminal"
 )
 
@@ -208,7 +210,13 @@ func encryptPass(pass string, in io.Reader, out io.Writer, armor bool) {
 func encrypt(recipients []age.Recipient, in io.Reader, out io.Writer, armor bool) {
 	ageEncrypt := age.Encrypt
 	if armor {
-		ageEncrypt = age.EncryptWithArmor
+		a := format.ArmoredWriter(out)
+		defer func() {
+			if err := a.Close(); err != nil {
+				logFatalf("Error: %v", err)
+			}
+		}()
+		out = a
 	}
 	w, err := ageEncrypt(out, recipients...)
 	if err != nil {
@@ -239,6 +247,14 @@ func decrypt(keys []string, in io.Reader, out io.Writer) {
 		identities = append(identities, ids...)
 	}
 
+	rr := bufio.NewReader(in)
+	armorHeader := "-----BEGIN AGE ENCRYPTED FILE-----"
+	if start, _ := rr.Peek(len(armorHeader)); string(start) == armorHeader {
+		in = format.ArmoredReader(rr)
+	} else {
+		in = rr
+	}
+
 	r, err := age.Decrypt(in, identities...)
 	if err != nil {
 		logFatalf("Error: %v", err)