data versioning concept
removed clean_html, cf https://github.com/nltk/nltk/issues/448
Revert "avoid conflict of nltk.data.load's 'format' argument with the builtin 'format' function", cf https://github.com/jskda/nltk/commit/edahttps://github.com/jskda/nltk/commit/9ec325935c5dc55e28844d9a70dac21c846fb987#commitcomment-3796092