Publish state to MQTT

[tomquist]

With this change, the script publishes the following info to the MQTT broker:

• The overall status of the script (online or offline). Automatically switches to offline when the script disconnects from the broker through an LWT
• The state of the current config: This is useful to acknowledge a config update and visualize initial config values in the home automation system
• The overall limit value
• The individual inverter limit values
• All log messages

To support all these MQTT features, I replaced the two topic configs with a single topic prefix config.