Reset all handlers. This allowing to use same server few times (#93)

* Reset all handlers. This allowing to use same server few times during app life.
E.g. for WiFi manager when hardware sepups WiFi connection and as HTTP server during hardware life cycle

README.md

@@ -807,6 +807,32 @@ server.on("/scan", HTTP_GET, [](AsyncWebServerRequest *request){
 });
 ```
 
+### Remove handlers and rewriters
+
+Server goes through handlers in same order as they were added. You can't simple add handler with same path to override them.
+To remove handler:
+```arduino
+// save callback for particular URL path
+auto handler = server.on("/some/path", [](AsyncWebServerRequest *request){
+  //do something useful
+});
+// when you don't need handler anymore remove it
+server.removeHandler(&handler);
+
+// same with rewriters
+server.removeRewrite(&someRewriter);
+
+server.onNotFound([](AsyncWebServerRequest *request){
+request->send(404);
+});
+
+// remove server.onNotFound handler
+server.onNotFound(NULL);
+
+// remove all writers and handlers, with onNotFound/onFileUpload/onRequestBody 
+server.reset();
+```
+
 ## Setting up the server
 ```cpp
 #include "ESPAsyncTCP.h"