Allow to override default template placeholder value with #define during compilation. (me-no-dev#366)

Signed-off-by: Alexandr Zarubkin <me21@yandex.ru>

Author: me21

src/WebResponseImpl.h

@@ -59,7 +59,10 @@ class AsyncAbstractResponse: public AsyncWebServerResponse {
     virtual size_t _fillBuffer(uint8_t *buf __attribute__((unused)), size_t maxLen __attribute__((unused))) { return 0; }
 };
 
+#ifndef TEMPLATE_PLACEHOLDER
 #define TEMPLATE_PLACEHOLDER '%'
+#endif
+
 #define TEMPLATE_PARAM_NAME_LENGTH 32
 class AsyncFileResponse: public AsyncAbstractResponse {
   using File = fs::File;