Option for replacing memory allocation and file I/O

[rationalcoder]

The STB style is good, but it's incomplete. There should be an option to provide file data without <stdio.h>and memory allocation without malloc()/realloc()/free(). See the STB libraries for examples.

[syoyo]

Yeah, its possible but require some amount of work(I want also remove other libc functions like sscanf)

PR is always welcome!