Add Dataflow support + Wikipedia + Wiki40b
# Add Dataflow support + Wikipedia + Wiki40b

## Support datasets processing with Apache Beam

Some datasets are too big to be processed on a single machine, for example: wikipedia, wiki40b, etc. Apache Beam allows to process datasets on many execution engines like Dataflow, Spark, Flink, etc.

To process such datasets with Beam, I added a command to run beam pipelines `nlp-cli run_beam path/to/dataset/script`. Then I used it to process the english + french wikipedia, and the english of wiki40b.
The processed arrow files are on GCS and are the result of a Dataflow job.

I added a markdown documentation file in `docs` that explains how to use it properly.

## Load already processed datasets

Now that we have those datasets already processed, I made it possible to load datasets that are already processed. You can do `load_dataset('wikipedia', '20200501.en')` and it will download the processed files from the Hugging Face GCS directly into the user's cache and be ready to use !

The Wikipedia dataset was already asked in #187 and this PR should soon allow to add Natural Questions as asked in #129 

## Other changes in the code

To make things work, I had to do a few adjustments:
- add a `ship_files_with_pipeline` method to the `DownloadManager`. This is because beam pipelines can be run in the cloud and therefore need to have access to your downloaded data. I used it in the wikipedia script:
    ```python
    if not pipeline.is_local():
            downloaded_files = dl_manager.ship_files_with_pipeline(downloaded_files, pipeline)
    ```
- add parquet to arrow conversion. This is because the output of beam pipelines are parquet files so we need to convert them to arrow and have the arrow files on GCS
- add a test script with a dummy beam dataset
- minor adjustments to allow read/write operations on remote files using `apache_beam.io.filesystems.FileSystems` if we want (it can be connected to gcp, s3, hdfs, etc...)