Add flag to enable custom implementation of nob_log

[isiloron]

Implementation of issue #137

Adds flag NOB_LOG_CUSTOM_IMPLEMENTATION.

When defined it will declare the function nob_log_default which has the same signature as nob_log.
If NOB_IMPLEMENTATION is defined, the nob_log definition will be renamed to nob_log_default.
If NOB_STRIP_PREFIX is defined, log_default will become an alias for nob_log_default.

The user has to define and implement nob_log on their own after including nob.h with the NOB_IMPLEMENTATION and NOB_LOG_CUSTOM_IMPLEMENTATION flags set.
The original nob_log implementation is still accessible by the user via nob_log_default.

Example:

// File: nob.c
#define NOB_IMPLEMENTATION
#define NOB_LOG_CUSTOM_IMPLEMENTATION
#include "nob.h"
#include "nob_extra.h"
#include "nob_bonus.h"

// Custom nob_log
void nob_log(Nob_Log_Level level, const char *fmt, ...)
{
    fprintf(stderr, "[CRITICAL ERROR] Urmom!\n");

    // Original nob_log accessible via nob_log_default
    va_list args;
    va_start(args, fmt);
    nob_log_default(level, fmt, args);
    va_end(args);
}

int main(int argc, char **argv)
{
    nob_log(NOB_INFO, "Start of build!");
    /*
    * The line above will print the following to stderr:
    * [CRITICAL ERROR] Urmom!
    * [INFO] Start of build!
    */

    nob_extra_foo();
    nob_bonus_bar();

    // <Rest of build goes here>
}

// File: nob_extra.c
#include "nob_extra.h"
#define NOB_LOG_CUSTOM_IMPLEMENTATION
#include "nob.h"

void nob_extra_foo()
{
    // Both lines below will print to stderr, the second line will circumvent the custom implementation.
    nob_log(LOG_INFO, "Hello custom logger from nob_extra_foo!");
    nob_log_default(LOG_INFO, "Hello default logger from nob_extra_foo!");
}

// File: nob_bonus.c
#include "nob_bonus.h"
#include "nob.h"

void nob_bonus_bar()
{
    /*
    * Of the lines below, only the first will compile without error.
    * In >=C99 (at least for gcc), the second line will most likely fail to compile
    * due to implicit function declaration being an error by default.
    * This is because NOB_LOG_CUSTOM_IMPLEMENTATION was not defined
    * before inclusion of nob.h
    */
    nob_log(LOG_INFO, "Hello custom logger from nob_bonus_bar!");
    nob_log_default(LOG_INFO, "Hello default logger from nob_bonus_bar!");
}