Add -stub flag to stub func implementation (#132)

When a mock is generated with the flag enabled, it introduces the
following changes:
- Does not panic on calling the method without a mock implementation.
- Return zero values iff the implementation is not provided and the
  method has return parameters.

Co-authored-by: Scott Leuthaeuser <scott_leuthaeuser@homedepot.com>

main.go

@@ -17,6 +17,7 @@ type userFlags struct {
 	outFile   string
 	pkgName   string
 	formatter string
+	stubImpl  bool
 	args      []string
 }
 
@@ -25,6 +26,8 @@ func main() {
 	flag.StringVar(&flags.outFile, "out", "", "output file (default stdout)")
 	flag.StringVar(&flags.pkgName, "pkg", "", "package name (default will infer)")
 	flag.StringVar(&flags.formatter, "fmt", "", "go pretty-printer: gofmt (default) or goimports")
+	flag.BoolVar(&flags.stubImpl, "stubImpl", false,
+		"return zero values when no mock implementation is provided, do not panic")
 
 	flag.Usage = func() {
 		fmt.Println(`moq [flags] source-dir interface [interface2 [interface3 [...]]]`)
@@ -59,6 +62,7 @@ func run(flags userFlags) error {
 		SrcDir:    srcDir,
 		PkgName:   flags.pkgName,
 		Formatter: flags.formatter,
+		StubImpl:  flags.stubImpl,
 	})
 	if err != nil {
 		return err