Add a WordPiece tokenizer layer

[mattdangerw]

Tensorflow text provides a set of efficient, in graph, WordPiece tokenization ops.

We would like to expose these through a keras layer in a way that is easily configurable, supports both tokenization and detokenization, and integrates properly with Keras functional models.

This can serve as an example for future subword tokenization work in this library.

[mattdangerw]

This is in as of #18