Datasets cli improvements
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Additionally, I've deleted the points that are not very relevant for this repo (I guess the deleted points originate from the transformers repo). With this change, running `datasets-cli` is identical to copy-pasting the code from `bug_report.md`, but is more elegant because it doesn't require launching the REPL and copy-pasting the code. 
This PR:
* replaces the code from the `bug_report.md` that was used to get relevant system info with a dedicated command (a more elegant approach than copy-pasting the code IMO)
* removes the `download` command (copied from the transformers repo?)
* adds missing help messages to the cli commands




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{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/2315', 'html_url': 'https://github.com/huggingface/datasets/pull/2315', 'diff_url': 'https://github.com/huggingface/datasets/pull/2315.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/2315.patch', 'merged_at': '2021-05-10T16:36:50Z'}