Specify config filename in HfArgumentParser
Author: jarednielsen
Pull request: yes

Currently, HfArgumentParser will load arguments from a config file if that config file is the same name as the script being run. So `train.py` would have a corresponding `train.args`. This extends the method to load from any config file that is specified. So `train.py` could use a `bert-large.args` or a `bert-small.args`.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6626?src=pr&el=h1) Report
> Merging [#6626](https://codecov.io/gh/huggingface/transformers/pull/6626?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/bc820476a5c72060f810f825298befd5ec85da4d?el=desc) will **decrease** coverage by `0.11%`.
> The diff coverage is `0.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6626/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6626?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6626      +/-   ##
==========================================
- Coverage   79.98%   79.87%   -0.12%     
==========================================
  Files         153      153              
  Lines       28005    28007       +2     
==========================================
- Hits        22401    22371      -30     
- Misses       5604     5636      +32     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6626?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/hf\_argparser.py](https://codecov.io/gh/huggingface/transformers/pull/6626/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9oZl9hcmdwYXJzZXIucHk=) | `67.74% <0.00%> (-1.49%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6626/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.47% <0.00%> (-32.95%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6626/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.44% <0.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6626/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `80.70% <0.00%> (+1.50%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6626/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `87.73% <0.00%> (+63.19%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6626?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/6626?src=pr&el=footer). Last update [bc82047...cf515d1](https://codecov.io/gh/huggingface/transformers/pull/6626?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Looks like a nice addition, LGTM

Thanks for adding this!