create callbacks to be able to overwrite file i/o functions

[hazemati]

one of the applicatiosn of this library is on embedded systems, where functions such as fopen cannot be taken from <stdio.h>. It'd be nice if we could leave the functions from stdio.h as default but give the user the option to overwrite them via callbacks (similiar to what you did with malloc and free).

[kgabis]

I'll look into it at some point.