Support directory removal

Currently, you're met with the following error (this is a `skrub-cli` call, but it makes no difference since it's an issue in this api)

``` sh
$ skrub <directory>
events.js:141
      throw er; // Unhandled 'error' event
      ^

Error: EISDIR: illegal operation on a directory, open '/Users/dawsonbotsford/<directory>'
    at Error (native)
```

(directory name redacted)
