Warning flag

Hello,

In your [03-Style.md](https://github.com/lefticus/cppbestpractices/blob/master/03-Style.md) best practice file you talk about using `nullptr` instead of `0` or `NULL` which is a very good things to do.

There is a flag in GCC that warn the user about using `0` (or `NULL`) instead of `nullptr`: `-Wzero-as-null-pointer-constant`. Unfortunately this flag does not exists for clang and I don't know if it does for MSVC.

I think this should be a good idea to add this flag in your warning flag list (https://github.com/lefticus/cppbestpractices/blob/master/02-Use_the_Tools_Available.md#gcc--clang).

Best regards