Add mg_log_set_fn(), MG_ENABLE_LOG

src/log.c

@@ -1,10 +1,19 @@
 #include "log.h"
 #include "util.h"
 
+static void default_logger(unsigned char c) {
+  putchar(c);
+}
+
 static const char *s_spec = "2";
+static void (*s_log_func)(unsigned char) = default_logger;
+
+void mg_log_set_callback(void (*fn)(unsigned char)) {
+  s_log_func = fn;
+}
 
 static void logc(unsigned char c) {
-  MG_PUTCHAR(c);
+  s_log_func(c);
 }
 
 static void logs(const char *buf, size_t len) {