Add support for user defined serial ports

[CullenSharp]

Summary

The ESP32 is capable to using custom pins for serial communication. This functionality should be supported by the library. For example, if I wanted to use GPIO16 and GPIO17 for serial communication via UART, I should be able to pass those pin numbers as arguments to the HLW8032.begin() method.

Tasks

• Update HLW8032.begin() prototype to accept GPIO pins as ints
• Update HLW8032.begin() source to implement this (see example)

See ESP32 UART Communication for details. The relevant snippet is below.

mySerial.begin(GPS_BAUD, SERIAL_8N1, RX_GPIO, TX_GPIO);