Partial Dataset Download

I am getting error when trying to download all the files within subdirectory of a competition data.

```
kaggle competitions download rsna-str-pulmonary-embolism-detection -f train/ID00007637202177411956430/
```

It would be great to have partial dataset download for quick prototyping outside kaggle.

Reproduce the competition download command shown in the issue and trace the CLI entry point that handles the -f path. Confirm how a directory path is interpreted; done means downloading all files beneath train/ID00007637202177411956430/ for quick prototyping, with coverage for the requested behavior.

Written by the indexing model from the issue text.