Add an option about git submodules to ignore

$GIT_STATUS_IGNORE_SUBMODULES can be used to specify handling of
submodules. It can be:
not set : ignore dirty submodules (this was default zsh behavior)
"git"   : do not use "--ignore-submodules" and let git choose,
          this obeys setting in .gitmodules
other   : comes into "--ignore-submodules=$GIT_STATUS_IGNORE_SUBMODULES"