Add post-checkout hook to remind user to update submodules

[sds]

When working in a repository with submodules, it would be nice to have a reminder that submodules have been updated.

This can be solved with a post-checkout hook that displays a warning if there are submodules that need updating. We'll probably want to have an option that controls whether we also check nested submodules (via the --recurse flag on git submodule status).

[sds]

Implemented by #164. Thanks!