Added IMGUI_DISABLE_DEFAULT_FILE_FUNCTIONS / IMGUI_DISABLE_FILE_FUNCTIONS #2734)

Using in Emscripten example.

imconfig.h

@@ -37,6 +37,7 @@
 //#define IMGUI_ENABLE_OSX_DEFAULT_CLIPBOARD_FUNCTIONS      // [OSX] Implement default OSX clipboard handler (need to link with '-framework ApplicationServices', this is why this is not the default).
 //#define IMGUI_DISABLE_DEFAULT_FORMAT_STRING_FUNCTIONS     // Don't implement ImFormatString/ImFormatStringV so you can implement them yourself (e.g. if you don't want to link with vsnprintf)
 //#define IMGUI_DISABLE_DEFAULT_MATH_FUNCTIONS              // Don't implement ImFabs/ImSqrt/ImPow/ImFmod/ImCos/ImSin/ImAcos/ImAtan2 so you can implement them yourself.
+//#define IMGUI_DISABLE_DEFAULT_FILE_FUNCTIONS              // Don't implement ImFileOpen/ImFileClose/ImFileRead/ImFileWrite so you can implement them yourself if you don't want to link with fopen/fclose/fread/fwrite. This will also disable the LogToTTY() function.
 //#define IMGUI_DISABLE_DEFAULT_ALLOCATORS                  // Don't implement default allocators calling malloc()/free() to avoid linking with them. You will need to call ImGui::SetAllocatorFunctions().
 
 //---- Include imgui_user.h at the end of imgui.h as a convenience