Add a NODE_ENV argument

I would like to set NODE_ENV like the following, because I cannot use `NODE_ENV=development` on Windows.

```bash
$ dotenv-flow -e development -- prisma migrate dev
```

I think the following implementation

```diff
  const path = argv.p;
- dotenvExpand.expand(dotenv.config({ path }));
+ const node_env = argv.e;
+ dotenvExpand.expand(dotenv.config({ path, node_env }));
```