Add 'keep_accents' flag to basic tokenizer
Hello!

Recently we released our Spanish Bert Model (https://github.com/dccuchile/beto) and we found problems with the tokenization for Spanish.
The problem relates to that the basic tokenizer convert the text to NFD.

For example:

```
text = "[CLS] compañera [SEP]"
tokenized_text = tokenizer.tokenize(text)
tokenized_text
['[CLS]', 'compa', '##ner', '##a', '[SEP]']
```

It changes *ñ* to *n*.

Another:

```
text = "[CLS] acción [SEP]"
tokenized_text = tokenizer.tokenize(text)
tokenized_text
['[CLS]',  'accion' ,'[SEP]']
```

It changes *ó* to *o*.

That behavior is not wanted for our Spanish model so in this PR I'm adding a flag to control that.

Waiting for your comments, thank you!
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2333?src=pr&el=h1) Report
> Merging [#2333](https://codecov.io/gh/huggingface/transformers/pull/2333?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/77b0a385ffac5964030d08b1c3611b61370b1918?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2333/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2333?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2333      +/-   ##
==========================================
+ Coverage   74.67%   74.67%   +<.01%     
==========================================
  Files          87       87              
  Lines       14800    14802       +2     
==========================================
+ Hits        11052    11054       +2     
  Misses       3748     3748
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2333?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/2333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `94.64% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/2333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `90.41% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `96.26% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG5ldC5weQ==) | `88.35% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/2333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `95.12% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `95.83% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.82% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9jdHJsLnB5) | `97.82% <ø> (ø)` | :arrow_up: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `87.6% <100%> (+0.04%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2333?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2333?src=pr&el=footer). Last update [77b0a38...386a104](https://codecov.io/gh/huggingface/transformers/pull/2333?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
