Add option for cloning/updating with submodules

[ardentsword]

I have a couple git submodules inside my repository, I would love to have an integrated option to clone/init those along with my main repository.

My current solution is to add a beforePreparing() step:

    public function beforePreparing()
    {
        $this->runRemote('cd {{ deploy_dir }}/repo; git submodule update --init --recursive;');
    }

This updates/initializes all submodules. It feels a bit hacky though, since this should really be in the doUpdateCode() function.