Consider being "Wzero-as-null-pointer-constant"-clean

[alabuzhev]

gcc 4.7 added a new command-line option -Wzero-as-null-pointer-constant to warn when a literal '0' is used as null pointer constant, which is useful to catch subtle errors as early as possible.

If this diagnostic is enabled project-wide, fmt produces a gazillion warnings because of things like Char *start = 0; in templates in the format.h. And since templates are instantiated in the user code, it's impossible to suppress this warning for fmt headers only.

Either nullptr or NULL should make gcc happy.
#define nullptr NULL is perfectly fine for pre-C++11 compilers.

[vitaut]

Using NULL sounds reasonable. Would you be willing to submit a PR?