Add CamemBERT models to modeling_auto
CamemBERT was in autoconfig but not in automodel, this PR aims to correct that. Also first PR here so please tell me if I missed some things :-)

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1902?src=pr&el=h1) Report
> Merging [#1902](https://codecov.io/gh/huggingface/transformers/pull/1902?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e70cdf083ddb8bfe298d43e6d70d698a3a2f56d3?src=pr&el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `14.28%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1902/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1902?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1902      +/-   ##
==========================================
- Coverage   84.08%   84.04%   -0.04%     
==========================================
  Files          97       97              
  Lines       14316    14323       +7     
==========================================
+ Hits        12037    12038       +1     
- Misses       2279     2285       +6
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1902?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/1902/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2F1dG8ucHk=) | `31.81% <14.28%> (-1.52%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1902?src=pr&el=footer). Last update [e70cdf0...75ef125](https://codecov.io/gh/huggingface/transformers/pull/1902?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Great, thanks @Evpok !