Add some scripts for dealing with git submodules.

These allow me to easily monitor and update them en masse.
list-submodules: Use the 'git submodule' command.

Instead of parsing the .gitmodules file directly, operate on the submodules that
have been initialized.
Add a script to update submodules with submodules.
Correct a path in the readme.
Merge branch 'submodules'