Add an event handler

[jonathanhliu21]

Is your feature request related to a problem? Please describe.
N/A

Describe the solution you'd like
Add an event handler to the library. For example, the user can create functions that are each run in separate threads whenever an event related to the serial port happens, such as when some data was received, when the serial port connects, or when the serial port disconnects.

Describe alternatives you've considered
N/A

Additional context
N/A

[jonathanhliu21]

Probably will do in the next major version (i.e. 1.x), so a very long time away