Mock code generation cli tool

Such as run the command below:

```bash
go run github.com/ysmood/got/cmd/mock@latest w myWriter io.Writer
```

To output generated golang code to the stdout:

```go
type myWriter struct {
	got.Mock
}

func (w *myWriter) Write(p []byte) (n int, err error) {
	return t.Proxy("Write").(func([]byte) (int, error))(p)
}
```