Code review comment for ~ballot/jenkins-plugin-manager/+git/jenkins-plugin-manager:master

The JSON files can be ignored because they are a snapshot of https://updates.jenkins.io/stable/update-center.json and https://updates.jenkins.io/plugin-versions.json

TODO in others MP:
* Download jenkins package from cli (only the library is implmented for now)
* Checksum of the downloaded package
* add various option in the "download" subcommand to allow optional dependencies and without if needed
* symlink the latest downloaded version if desired
* document the format of the plugin --from-file format