Add option to use a 'stop token'
This will be used to truncate the output text to everything till right before the 'stop token'. If the 'stop token' is not found, then the whole text will be returned based on the specified 'length'.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1313?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@ecc4f1b`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1313/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1313?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master    #1313   +/-   ##
=========================================
  Coverage          ?   84.72%           
=========================================
  Files             ?       84           
  Lines             ?    12591           
  Branches          ?        0           
=========================================
  Hits              ?    10668           
  Misses            ?     1923           
  Partials          ?        0
```



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<|||||>Great, thanks!