Sub-modules for extensions #158

Have you ever thought of using Git submodules for the various extensions?

This has several advantages, among them:

• you can easily download only the extensions you are interested in
• you can work on them more easily (you just need to clone the specific repository, not all of them
• Git history would be cleaner and more readable
• when you add a new extension, this will not affect at all the repositories of existing ones (for instance, Git history)
• ...