Adding utility functions - IsNameOnly and WithDefaultTag

Signed-off-by: Nishant Totla <nishanttotla@gmail.com>

reference/helpers.go

@@ -0,0 +1,12 @@
+package reference
+
+// IsNameOnly returns true if reference only contains a repo name.
+func IsNameOnly(ref Named) bool {
+	if _, ok := ref.(NamedTagged); ok {
+		return false
+	}
+	if _, ok := ref.(Canonical); ok {
+		return false
+	}
+	return true
+}