Wizard should handle git submodules

[adehnert]

Sometimes we want to autoinstall an app that uses submodules, or an app that supports extensions and we want to preinstall some extensions. It would be nice to be able to stick the extension in a submodule, rather than combining it into the main git repo. Wizard should support this approach. (Edward says it doesn't.)

(I don't really care about having any useful integration besides "keep the submodule up-to-date".)