Add -rm flag to remove target output file (#151)

File is removed before mock generation, if it exists. This is useful
when the mock generation is likely to fail due to some reason, for 
example, package load failure due to change in the interface being
mocked (the existing mock is no longer valid and cannot be compiled). In
such cases, the -rm flag can be used instead of manually removing the
file (which could have fixed the issue).

main.go

@@ -22,6 +22,7 @@ type userFlags struct {
 	formatter  string
 	stubImpl   bool
 	skipEnsure bool
+	remove     bool
 	args       []string
 }
 
@@ -35,6 +36,7 @@ func main() {
 	printVersion := flag.Bool("version", false, "show the version for moq")
 	flag.BoolVar(&flags.skipEnsure, "skip-ensure", false,
 		"suppress mock implementation check, avoid import cycle if mocks generated outside of the tested package")
+	flag.BoolVar(&flags.remove, "rm", false, "first remove output file, if it exists")
 
 	flag.Usage = func() {
 		fmt.Println(`moq [flags] source-dir interface [interface2 [interface3 [...]]]`)
@@ -63,6 +65,14 @@ func run(flags userFlags) error {
 		return errors.New("not enough arguments")
 	}
 
+	if flags.remove && flags.outFile != "" {
+		if err := os.Remove(flags.outFile); err != nil {
+			if !errors.Is(err, os.ErrNotExist) {
+				return err
+			}
+		}
+	}
+
 	var buf bytes.Buffer
 	var out io.Writer = os.Stdout
 	if flags.outFile != "" {