text: Title: Downloader fails when download_dir is a nested folder that doesn't exist
  Body: ### Problem
  When the download folder doesn't exist, it is created by the downloader. But when that folder is nested, the creation fails.
  
  ### Code example
  
  ```python
  nltk.download("punkt", force=True, download_dir="some/nested/folder")
  ```
  
  Will give the following error:
  
  ```
  File "my_env/lib/python3.7/site-packages/nltk/downloader.py", line 776, in download
      for msg in self.incr_download(info_or_id, download_dir, force):
    File "my_env/lib/python3.7/site-packages/nltk/downloader.py", line 641, in incr_download
      yield from self._download_package(info, download_dir, force)
    File "my_env/lib/python3.7/site-packages/nltk/downloader.py", line 698, in _download_package
      os.mkdir(download_dir)
  FileNotFoundError: [Errno 2] No such file or directory: 'some/nested/folder'
  ```
  
  ### Suggested solution
  
  The following line throws the error: https://github.com/nltk/nltk/blob/develop/nltk/downloader.py#L698
  
  `os.makedirs(download_dir)` could be used instead of `os.mkdir(download_dir)`
  
  