Additional categories for different NLTK usages

[alvations]

We have all-corpora and all but it'll be nice if we can several new category that includes:

• popular

  • punkt
  • stopwords
  • wordnet
  • averaged_perceptron_tagger
  • brown
  • movie_reviews
  • words

• tokenizers

  • punkt
  • snowball
  • perluniprops
  • nonbreaking_prefixes

That way I think it's easier to advise users to do the following to install nltk:

pip install -U nltk
python -m nltk.downloader popular

More importantly, I think all-no-third-party and all-third-party, so that we can separate issues when the third-party datasets/models don't update their checksum to nltk when they refresh their data/models.

@stevenbird Are the suggestions okay? How should we go about adding these categories?

[stevenbird]

@alvations: great idea; simply create new collections over in nltk_data/collections/