add config support

includes/tomlc17-R260618/OPTIONS.md

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+# tomlc17 Options
+
+## Setting Options: Custom Allocators
+
+By default, `tomlc17` uses the standard C library `realloc()` and `free()` 
+functions for memory management. You can override this behavior by setting 
+custom allocator functions using `toml_set_option()`.
+
+This is useful when you need to:
+- Use a custom memory pool or arena allocator
+- Track memory allocations for debugging
+- Enforce memory constraints in embedded or resource-limited environments
+- Integrate with a specialized memory management system
+
+### Example: Using Custom Allocators
+
+```c
+#include "tomlc17.h"
+#include <stdlib.h>
+
+// Custom realloc function
+void* my_realloc(void *ptr, size_t size) {
+  // Your custom allocation logic here
+  if (size == 0) {
+    free(ptr);
+    return NULL;
+  }
+  return realloc(ptr, size);
+}
+
+// Custom free function
+void my_free(void *ptr) {
+  // Your custom deallocation logic here
+  free(ptr);
+}
+
+int main() {
+  // Set custom allocators
+  toml_option_t opt = toml_default_option();
+  opt.mem_realloc = my_realloc;
+  opt.mem_free = my_free;
+  toml_set_option(opt);
+
+  // Now all subsequent parse calls will use your custom allocators
+  toml_result_t result = toml_parse_file_ex("config.toml");
+  
+  if (result.ok) {
+    // Use the parsed data
+    toml_free(result);
+  }
+
+  return 0;
+}
+```
+
+### Important Notes
+
+- **Initialization:** Call `toml_set_option()` once during program startup, before 
+  any parsing operations.
+- **Thread Safety:** `toml_set_option()` is not thread-safe. Set options only during 
+  initialization in a single-threaded context.
+- **Custom Allocator Requirements:** Your custom `mem_realloc` function should behave 
+  like standard `realloc()`:
+  - When `size > 0`: allocate or resize memory, return pointer or NULL on failure
+  - When `size == 0`: free memory and return NULL
+  - Your custom `mem_free` function should handle NULL pointers gracefully
+
+For more details, see the [`toml_set_option()` documentation](API.md#toml_set_option) 
+in API.md.