__RLTK__

An attempt to manually port some of nltk to rust.

Currently in it's infancy: 
* rltk::lm::preprocessing::pad_both_ends(\["a","b","c"], 2) -> "\<s>", "a", "b", "c", "\</s>"]
* rltk::util::pad_sequence == same as above with customisation
* rltk::util::pad_sequence_left == same
* rltk::util::pad_sequence_right == same
* rltk::util::ngrams(\["a","b","c"],2) -> \[\["a"], \["b"], \["b"], \["c"]]
* rltk::util::bigrams(\["a","b","c"]) == ngrams(..., 2) 
* rltk::util::trigrams(\["a","b","c"]) == ngrams(..., 3)