Improve argument parsing, fix style

[jcfr]

In addition of fixing some stylistic issues, this PR also implements the following features:

• proper parsing of command line argument for both github-release and github-asset executable
• support for GITHUB_TOKEN environment variable as an alternative to the ~/.netrc
• support for "single cli" usage either as an installed executable (githubrelease) or directly using github_release.py script:

$ githubrelease
usage: githubrelease [-h] {release, asset} ...

positional arguments:
    {release, asset}
                        sub-command help
    release             Manage releases (list, create, delete, ...)
    asset               Manage release assets (upload, download, ...)

optional arguments:
  -h, --help            show this help message and exit
githubrelease: error: too few arguments

$ githubrelease release
usage: githubrelease release [-h]
                             repo_name
                             {info,release-notes,debug,create,unpublish,list,publish,delete}
                             ...
githubrelease release: error: too few arguments

$ githubrelease release --help
usage: githubrelease release [-h]
                             repo_name
                             {info,release-notes,debug,create,unpublish,list,publish,delete}
                             ...

positional arguments:
  repo_name
  {info,release-notes,debug,create,unpublish,list,publish,delete}
                        sub-command help
    info                Get release description
    release-notes       Set release notes
    debug               Print release detailed information
    create              Create a release
    unpublish           Unpublish a release setting draft to 'True'
    list                List releases
    publish             Publish a release setting draft to 'False'
    delete              Delete a release

optional arguments:
  -h, --help            show this help message and exit

$ githubrelease asset
usage: githubrelease asset [-h] repo_name {download,erase,upload,delete} ...
githubrelease asset: error: too few arguments

$ githubrelease asset --help
usage: githubrelease asset [-h] repo_name {download,erase,upload,delete} ...

positional arguments:
  repo_name
  {download,erase,upload,delete}
                        sub-command help
    download            Download release assets
    erase               Delete release assets
    upload              Upload release assets
    delete              Delete release assets

optional arguments:
  -h, --help            show this help message and exit

[jcfr]

@j0057 @jnweiger This PR should address some of the usability concern.

If you prefer, I could split the PR in multiple PRs. If the proposed the change looks good, I will look into updating the documentation.