BertTokenizerFast does not support `pad_to_max_length` argument
# 🐛 Bug

The fast tokenizer has different behavior from the normal tokenizer.

```python
from transformers import BertTokenizer, BertTokenizerFast

BertTokenizer.from_pretrained("bert-base-uncased").encode("hello world", max_length=128, pad_to_max_length="right")
# succeeds
BertTokenizerFast.from_pretrained("bert-base-uncased").encode("hello world", max_length=128, pad_to_max_length="right")
*** TypeError: enable_padding() got an unexpected keyword argument 'max_length'
```

## Environment info
     
- `transformers` version: 2.11.0
- `tokenizers` version: 0.8.0rc3
- Platform: Ubuntu 18.04
- Python version: 3.7

Hi @jarednielsen, if you installed from source then padding is handled in a different way. You'll need to use the newly added `padding` argument. According to the docs 

`padding` (:obj:`Union[bool, str]`, `optional`, defaults to :obj:`False`):
            Activate and control padding. Accepts the following values:
    
            * `True` or `'longest'`: pad to the longest sequence in the batch (or no padding if only a single sequence if provided),
            * `'max_length'`: pad to a max length specified in `max_length` or to the max acceptable input length for the model if no length is provided (`max_length=None`)
            * `False` or `'do_not_pad'` (default): No padding (i.e. can output batch with sequences of uneven lengths)
<|||||>Yes, this works on master (both the old and new tokenizer API) and should work in the new release that will be out very soon.<|||||>Thank you for the quick response! Reading https://github.com/huggingface/transformers/pull/4510 makes it much clearer.<|||||>Yes, we even have a nice tutorial on the new tokenizer API now thanks to the amazing @sgugger:
https://huggingface.co/transformers/master/preprocessing.html