Make minisign optional

Given that age is AEAD, additional MAC would be redundant,
so we keep minisign as optional to where it fits the use case.

Fixes #2

src/fopus.sh

@@ -331,6 +331,10 @@ split_file()
 sign_files()
 {
 	if [[ "$DRY_RUN" == "false" ]]; then
+		if [[ -z "$MINISIGN_KEY_PATH" ]]; then
+			return 0
+		fi
+
 		# hash
 		(
 		cd "$BACKUP_PATH/$BACKUP_DIR" || exit 1