add BertTokenizer flag to skip basic tokenization
When tokenization is done before text hits this package (e.g., when tokenization is specified as part of the dataset) there exists a use case for skipping the `BasicTokenizer` step, going right to `WordpieceTokenizer`.

When one still wants to use the `BertTokenizer.from_pretrained` helper function, they have been able to do this (without claiming this is necessarily the best way) by

```
text = "[CLS]  `` Truly , pizza is delicious , '' said Mx. Caily."
bert_tokenizer = BertTokenizer.from_pretrained('bert-base-cased')
tokenized_text = bert_tokenizer.wordpiece_tokenizer.tokenize(text)
```

With this PR, we instead use
```
text = "[CLS]  `` Truly , pizza is delicious , '' said Mx. Caily."
bert_tokenizer = BertTokenizer.from_pretrained('bert-base-cased', do_basic_tokenize=False)
tokenized_text = bert_tokenizer.tokenize(text)
```

a flag for which I add documentation in the docstring and README, hopefully making it clear that this is possible. 

Thanks for the PR (and documenting this), I added a note<|||||>Ok this is great, thanks @john-hewitt, thanks!