Automatically use --unit B when using --bytes

Hi,

`tqdm` is very useful in `tar | gz` pipes when used like:

```bash
tar c $path | tqdm --unit B --bytes --unit_scale --total $(du -bs $path | cut -f1) | gzip $path
```

It would be nice if `--unit B` could be used automatically when `--bytes` is used.

Thanks for your work on tqdm!