Add fmt flag to specify formatter (#117)

- Support formatting output with gofmt(default) or goimports via flag.
- Introduce moq.Config struct to configure moq.Mocker instance. This
  breaks backward compatibility but facilitates it in the future if and
  when any features are added.
- Use golden file tests for validating formatters. Use 
  github.com/pmezard/go-difflib to print the diff between expected and
  actual.

main.go

@@ -14,18 +14,20 @@ import (
 )
 
 type userFlags struct {
-	outFile string
-	pkgName string
-	args    []string
+	outFile   string
+	pkgName   string
+	formatter string
+	args      []string
 }
 
 func main() {
 	var flags userFlags
 	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.Usage = func() {
-		fmt.Println(`moq [flags] destination interface [interface2 [interface3 [...]]]`)
+		fmt.Println(`moq [flags] source-dir interface [interface2 [interface3 [...]]]`)
 		flag.PrintDefaults()
 		fmt.Println(`Specifying an alias for the mock is also supported with the format 'interface:alias'`)
 		fmt.Println(`Ex: moq -pkg different . MyInterface:MyMock`)
@@ -52,9 +54,12 @@ func run(flags userFlags) error {
 		out = &buf
 	}
 
-	destination := flags.args[0]
-	args := flags.args[1:]
-	m, err := moq.New(destination, flags.pkgName)
+	srcDir, args := flags.args[0], flags.args[1:]
+	m, err := moq.New(moq.Config{
+		SrcDir:    srcDir,
+		PkgName:   flags.pkgName,
+		Formatter: flags.formatter,
+	})
 	if err != nil {
 		return err
 	}