[feature request] custom printf format specifiers

This would enable custom **printf** implementations to do more and also be able to rely on `-Wformat` or `-Werror=format` flag.

Here's initial suggestion:

```c
/* "int" types would implicitly also define "%lb", "%llb" and "%zb" */
#pragma clang format "%b" "int"

typedef struct string_s {
    char* buf;
    size_t size;
} string_t;

/* override */
#pragma clang format "%s" "string_t"
```
