Add decryptMime and refactor package structure

internal/armor.go

@@ -0,0 +1,16 @@
+package internal
+
+import (
+	"golang.org/x/crypto/openpgp/armor"
+	"strings"
+)
+
+func UnArmor(input string) (*armor.Block, error) {
+	io := strings.NewReader(input)
+	b, err := armor.Decode(io)
+	if err != nil {
+		return nil, err
+	}
+	return b, nil
+
+}