Ngrams destroy punctuation

[giorgio79]

Example:
Would be nice to have an option that preserves punctuation:

console.log(nautral_NGrams.bigrams('Some, words here!!'));
[ [ 'Some', 'words' ], [ 'words', 'here' ] ]

I would have liked to see
[ [ 'Some,', 'words' ], [ 'words', 'here!!' ] ]

If chaining commands is implemented eventually at #439 than one could just strip punctuation previously, or pass in to tokenizator first.

[giorgio79]

Also, tokenizers already split the text in various ways, so I would just keep the splitting logic with the tokenizers...