Add an option to automatically submodule update on checkout

[Ciro Santilli]

This is a must if you are working with submodules, otherwise every git
checkout requires a git submodule update, and you forget it, and
things break, and you understand, and you go to stack overflow
questions 
http://stackoverflow.com/questions/22328053/why-doesnt-git-checkout-automatically-do-git-submodule-update-recursive
| 
http://stackoverflow.com/questions/4611512/is-there-a-way-to-make-git-pull-automatically-update-submodules
and you give up and create aliases :-)