Add requestHandler (fixes #119)

docs/web-server.md

@@ -14,6 +14,27 @@ The web server has only one public method. Call this method in your setup functi
 
 ## Class Members
 
+#### requestHandler
+```c++
+ArRequestHandlerFunction requestHandler = serveProgmem;
+```
+
+Use this to override the requestHandler before calling `GUI.begin()`:
+
+```c++
+GUI.requestHandler = alternativeRequest; //override request handler
+GUI.begin();
+```
+With another request handler, such as:
+
+```c++
+void alternativeRequest(AsyncWebServerRequest *request)
+{
+    request->send(200, "text/plain", "Hello World!");
+}
+```
+This allows you to add your own logic into the webserver.
+
 #### server
 
 ```c++

src/webServer.cpp

@@ -21,7 +21,7 @@ void webServer::begin()
 
     server.serveStatic("/download", LittleFS, "/");
 
-    server.onNotFound(serveProgmem);
+    server.onNotFound(requestHandler);
 
     //handle uploads
     server.on(PSTR("/upload"), HTTP_POST, [](AsyncWebServerRequest *request) {}, handleFileUpload);

src/webServer.h

@@ -14,6 +14,7 @@ class webServer
 public:
     AsyncWebServer server = AsyncWebServer(80);
     AsyncWebSocket ws = AsyncWebSocket("/ws");
+    ArRequestHandlerFunction requestHandler = serveProgmem;
     void begin();
 };