Adds WiFi scanner

Adds _optional field named "options" when defining configuration options. setting it to uEspConfigLib_OPTION_SCANNER adds a WiFi scanner link that opens a new window listing all WiFis and lets you to select one SSID on that new window. This only works on HTML config page.

A typica usage is:

config->addOption("wifi_ssid", "SSID of your WiFi", "Unconfigured_device", uEspConfigLib_OPTION_SCANNER);