Add URL for version control systems

[cornelius]

We already have a URL pointing to the development source code, which usually is a git repository. The URL usually links to the web page showing the repo in a human-readable way.

To make it easier to integrate with version control system tooling, it would be nice to have a pointer in the meta data, which can be directly used by git (or any other version control system used for managing the library's source code). This could be something like

"vcs": { "git": { "clone_url": "..." } } }