Allow access to underlying n-gram tokenizer

[Vic Kumar]

The N-gram implementation: https://github.com/vickumar1981/stringdistance/blob/master/src/main/scala/com/github/vickumar1981/stringdistance/impl/NGramImpl.scala

• can allow access to the tokens, such that a function: Ngram.tokens("something", 2)
  willresult in an Array[String] which are the n-grams themselves.

• the function to do this is here: https://github.com/vickumar1981/stringdistance/blob/master/src/main/scala/com/github/vickumar1981/stringdistance/interfaces/NGramTokenizer.scala#L9

[Vic Kumar]

Added functions to n-gram classes: #46

[Vic Kumar]

Included in version 1.2.0