[WIP] Optional whitespace cleansing for CountVectorizer(analyzer='char')

Currently, getting character n-grams from CountVectorizer (or variants) automatically normalises whitespace, and there is not current option to turn it off. This PR adds this option.

Pretty basic PR here: `normalize_whitespace` option to CountVectorizer (and variants), True by default, which matches current behaviour. When set to False, the normalisation that occurred in `CountVectorizer._char_ngrams` no longer happens.

Current todo:
[ ] Test case
[ ] Narrative documentation updates if applicable