Auto-setup submodules

[zml2008]

While we don't want to unconditionally sync submodules with what's committed, it would be helpful to initialize submodules that are not yet initialized (i.e. when a user cloned non-recursively, as in SpongePowered/Configurate#302

We could instead fail with a friendly error message, and/or always log a warning when the submodule is checked out to something other than the committed ref?

is this worth being configurable?