Fix tensorflow_dataset glue support
This PR fixes issue #1354 .

`glue_convert_examples_to_features` assumed that tensorflow_dataset examples contains the features `'sentence1'` and `'sentence2'`. This commit encapsulates the choice of features in the glue processor and uses that to parse examples.

Built with @philipp-eisen .
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1355?src=pr&el=h1) Report
> Merging [#1355](https://codecov.io/gh/huggingface/transformers/pull/1355?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ca559826c4188be8713e46f191ddf5f379c196e7?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `50%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1355/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1355?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1355      +/-   ##
==========================================
- Coverage   84.73%   84.68%   -0.05%     
==========================================
  Files          84       84              
  Lines       12573    12592      +19     
==========================================
+ Hits        10654    10664      +10     
- Misses       1919     1928       +9
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1355?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/data/processors/utils.py](https://codecov.io/gh/huggingface/transformers/pull/1355/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2RhdGEvcHJvY2Vzc29ycy91dGlscy5weQ==) | `46.66% <100%> (+1.21%)` | :arrow_up: |
| [transformers/data/processors/glue.py](https://codecov.io/gh/huggingface/transformers/pull/1355/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2RhdGEvcHJvY2Vzc29ycy9nbHVlLnB5) | `27.98% <47.36%> (+1.76%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1355?src=pr&el=footer). Last update [ca55982...795b3e7](https://codecov.io/gh/huggingface/transformers/pull/1355?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Nice and clean, thanks a lot @agrinh and @philipp-eisen!<|||||>@thomwolf Happy to help, we're finding this package super useful!