Add config and mode parameters to enableSerial() call

[rlerdorf]

Since the 8266 has so few IO pins, it is quite common to use the RX pin for IO. To do so it is a good idea to initialize the UART using:

Serial.begin(115200, SERIAL_8N1, SERIAL_TX_ONLY);

But iot->enableSerial() didn't support that. This adds those two optional params.

[coogle]

Thanks for the commit!