CLI for authenticated file sharing
Author: julien-c
Pull request: yes

ping review @mfuntowicz & @thomwolf 

(I'll fix the tests for Python 2 and Python 3.5 tomorrow)


To create an account in `staging` (used by the tests): https://moon-staging.huggingface.co/join
To create an account in `production` (used by the CLI): https://huggingface.co/join

Comments:
Seen in person with @julien-c, really slick implementation!

Can't wait to test it 😊

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2044?src=pr&el=h1) Report
> Merging [#2044](https://codecov.io/gh/huggingface/transformers/pull/2044?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7edb51f3a516ca533797fb2bb2f2b7ce86e0df70?src=pr&el=desc) will **decrease** coverage by `0.33%`.
> The diff coverage is `50.46%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2044/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2044?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2044      +/-   ##
==========================================
- Coverage   83.45%   83.12%   -0.34%     
==========================================
  Files         105      109       +4     
  Lines       15568    15784     +216     
==========================================
+ Hits        12993    13121     +128     
- Misses       2575     2663      +88
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2044?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/commands/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/2044/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2NvbW1hbmRzL19faW5pdF9fLnB5) | `0% <0%> (ø)` | |
| [transformers/commands/user.py](https://codecov.io/gh/huggingface/transformers/pull/2044/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2NvbW1hbmRzL3VzZXIucHk=) | `0% <0%> (ø)` | |
| [transformers/hf\_api.py](https://codecov.io/gh/huggingface/transformers/pull/2044/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2hmX2FwaS5weQ==) | `96.87% <96.87%> (ø)` | |
| [transformers/tests/hf\_api\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2044/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL2hmX2FwaV90ZXN0LnB5) | `97.91% <97.91%> (ø)` | |
| [transformers/tests/modeling\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2044/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX2NvbW1vbl90ZXN0LnB5) | `74.23% <0%> (+0.19%)` | :arrow_up: |
| [transformers/tests/modeling\_bert\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2044/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX2JlcnRfdGVzdC5weQ==) | `96.5% <0%> (+0.5%)` | :arrow_up: |
| ... and [14 more](https://codecov.io/gh/huggingface/transformers/pull/2044/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2044?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2044?src=pr&el=footer). Last update [7edb51f...3ba417e](https://codecov.io/gh/huggingface/transformers/pull/2044?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


If anyone wants to try it out before it's merged, instructions are:
```bash
git checkout cli_upload
pip install -e .
transformers-cli login
transformers-cli upload
```

Perfect, I love it! Very slick