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.

internal/format/format.go

@@ -112,13 +112,6 @@ func Parse(input io.Reader) (*Header, io.Reader, error) {
 	h := &Header{}
 	rr := bufio.NewReader(input)
 
-	// TODO: find a way to communicate to the caller that the file was armored,
-	// as they might not appreciate the malleability.
-	if start, _ := rr.Peek(len(armorPreamble)); string(start) == armorPreamble {
-		input = ArmoredReader(rr)
-		rr = bufio.NewReader(input)
-	}
-
 	line, err := rr.ReadString('\n')
 	if err != nil {
 		return nil, nil, errorf("failed to read intro: %v", err)