Add submodule argument for line_limit.sh

Add -s, --submodules argument to line_limit.sh.

When specified:

• Check submodule paths normally

When not specified:

• Check if .gitmodules exists
• Parse submodule paths
• Exclude paths from check list automatically

For this to work properly, the script's PWD will need to be the root of the git repository it is run from. Add a note about this in the help.