Feature request: re-open AP to change configurations after boot

[willblev]

Currently, once the configuration has been set, the WiFiManager library will load this and connect automatically after boot. However, we can use some sequence of button presses or a special Slack status to re-open the AP config web portal.

Here's an example

//set callback that gets called when connecting to previous WiFi fails, and enters Access Point mode
wifiManager.setAPCallback(configModeCallback);


//gets called when WiFiManager enters configuration mode
void configModeCallback (WiFiManager *myWiFiManager) {
  Serial.println("Entered config mode");
  Serial.println(WiFi.softAPIP());
  //if you used auto generated SSID, print it
  Serial.println(myWiFiManager->getConfigPortalSSID());
}