Configure default generator with no config

When no config file is specified while using new configuration style,
assume some code generation defaults, namely (client, server, echo, embed),
to be consistent with old default behavior.

cmd/oapi-codegen/oapi-codegen.go

@@ -198,6 +198,21 @@ func main() {
 			if err != nil {
 				errExit("error parsing'%s' as YAML: %v\n", flagConfigFile, err)
 			}
+		} else {
+			// In the case where no config file is provided, we assume some
+			// defaults, so that when this is invoked very simply, it's similar
+			// to old behavior.
+			opts = configuration{
+				Configuration: codegen.Configuration{
+					Generate: codegen.GenerateOptions{
+						EchoServer:   true,
+						Client:       true,
+						Models:       true,
+						EmbeddedSpec: true,
+					},
+				},
+				OutputFile: flagOutputFile,
+			}
 		}
 		var err error
 		opts, err = updateConfigFromFlags(opts)