Handle simple configs with output flag properly

It turns out that a very simple command invocation, specifying only
the spec and `-o` flag to generate an output filename ignores the
output filename option.

cmd/oapi-codegen/oapi-codegen.go

@@ -21,9 +21,10 @@ import (
 	"runtime/debug"
 	"strings"
 
+	"gopkg.in/yaml.v2"
+
 	"github.com/deepmap/oapi-codegen/pkg/codegen"
 	"github.com/deepmap/oapi-codegen/pkg/util"
-	"gopkg.in/yaml.v2"
 )
 
 func errExit(format string, args ...interface{}) {
@@ -334,6 +335,10 @@ func updateConfigFromFlags(cfg configuration) (configuration, error) {
 			strings.Join(unsupportedFlags, ", "))
 	}
 
+	if cfg.OutputFile == "" {
+		cfg.OutputFile = flagOutputFile
+	}
+
 	return cfg, nil
 }