Add tooling for maintaining an extension's GitHub repository

[aldesantis]

When developers create a new extension, they not only have to set up the file structure, but also the Git/GitHub repository. All extension GitHub repositories share the same settings, and it would be useful to allow an extension maintainer to automatically create and configure a repository via a CLI executable provided by solidus_dev_support.

• Allow to create a local git repository
• Allow to also create a remote GH repository
• Allow to configure the remote repo
• Propose a standard way to notify github.com/solidusio-contrib maintainers about the new extension (an awesome-solidus list?)
• Give clear instructions on how to move the extension under solidusio-contrib

[chrean]

Extensions are not created very often, so I would close this one for now.