Print to stdout by default

Thanks for this tool! I just wanted to suggest that perhaps `pipreqs` print to stdout by default, allowing you to redirect to a file via:

```bash
pipreqs > requirements.txt
```

But this would let you check the results before writing (or overwriting) to the file, by seeing it in your terminal first. Another option is to use a `-o requirements.txt` flag.

Just a suggestion :)