Add placeholder-name CLI

- main and unit tests for main
- client package to be done in a future commit

Signed-off-by: Aram Price <pricear@vmware.com>

internal/constable/error.go

@@ -0,0 +1,14 @@
+/*
+Copyright 2020 VMware, Inc.
+SPDX-License-Identifier: Apache-2.0
+*/
+
+package constable
+
+var _ error = Error("")
+
+type Error string
+
+func (e Error) Error() string {
+	return string(e)
+}