Topic-specific callback methods

This release adds:

• add_topic_callback method: Registers a callback_method for a specific MQTT topic.
• remove_topic_callback method: Removes a registered callback method
  • Usage example

To use in CircuitPython, simply install the Adafruit CircuitPython bundle.

To use in CPython, pip3 install adafruit-circuitpython-minimqtt.

Read the docs for info on how to use it.