[multiple-choice] Simplify and use tokenizer.encode_plus
Our base tokenizer `PreTrainedTokenizer` now has the ability to encode a sentence pair up to a `max_length`, adding special tokens for each model and returning a mask of `token_type_ids`.

In this PR we upgrade `run_multiple_choice` by adopting this factorized tokenizer API.

To ensure the results are strictly the same as before, we implement a new `TruncatingStrategy` (ideally this could be an enum).

@erenup as you spent a lot of time on this script, would you be able to review this PR?

Result of eval with parameters from [examples/readme](https://github.com/huggingface/transformers/blob/julien_multiple-choice/examples/README.md#multiple-choice):
```
eval_acc = 0.8352494251724483
eval_loss = 0.42866929549320487
```
Great addition. I feel like using enums would be especially helpful for the truncating strategy, indeed.<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1385?src=pr&el=h1) Report
> Merging [#1385](https://codecov.io/gh/huggingface/transformers/pull/1385?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5c3b32d44d0164aaa9b91405f48e53cf53a82b35?src=pr&el=desc) will **decrease** coverage by `0.07%`.
> The diff coverage is `41.66%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1385/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1385?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1385      +/-   ##
==========================================
- Coverage   84.69%   84.61%   -0.08%     
==========================================
  Files          84       84              
  Lines       12596    12610      +14     
==========================================
+ Hits        10668    10670       +2     
- Misses       1928     1940      +12
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1385?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1385/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rva2VuaXphdGlvbl91dGlscy5weQ==) | `87.73% <41.66%> (-2.46%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1385?src=pr&el=footer). Last update [5c3b32d...9e136ff](https://codecov.io/gh/huggingface/transformers/pull/1385?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>I have reviewed this PR and It looks good to me. Thank you! @julien-c . I added two lines of comments above. Hope they are useful. Thank you. <|||||>Merged in, and superseded by, #1384