Add better MQTT functionality

[makermelissa]

This adds the on_mqtt_unsubscribe event. It also adds error handling for the publish command to handle an error @jedgarpark was running into:

File “code.py”, line 78, in <module>
  File “adafruit_io/adafruit_io.py”, line 413, in publish
  File “adafruit_minimqtt/adafruit_minimqtt.py”, line 618, in publish
OSError: [Errno 104] ECONNRESET

[brentru]

I do not understand this method's purpose - is it exposing the MQTT client's publish method and providing error handling? Why not directly call the MQTT client's publish() through this method, passing in its expected arguments instead of kwargs?

[makermelissa]

That's correct, but I did it this way because this works with both mini mqtt and io mqtt and the publish methods have different arguments. See:
https://github.com/adafruit/Adafruit_CircuitPython_MiniMQTT/blob/master/adafruit_minimqtt/adafruit_minimqtt.py#L556
and
https://github.com/adafruit/Adafruit_CircuitPython_AdafruitIO/blob/master/adafruit_io/adafruit_io.py#L372

[brentru]

Ok, I'll leave it for now...I disagree with using a "global" function for publishing via two separate libraries, users may get confused between the syntax of providing a feed key and an MQTT topic (differently formatted strings).

[brentru]

Ideally, we'd have io_publish and mqtt_publish to really split it out. Each method would mirror the publish method to its corresponding library.

[makermelissa]

Fair enough. I was trying to keep the code from growing too big, but may eventually end up going that route.