Improved devicePublish topic parsing

[Zagitta]

I have refactored the devicePublish topic parsing to use regex capture groups as this seems more natural than manually splitting on slashes.
Furthermore I have added the ability to set attributes directly using the mqtt topic rather than having to set it using JSON while still maintaining backwards compatibility with the JSON approach.
This was mainly done to enable thermostat integration with homeassistant's MQTT climate component: https://www.home-assistant.io/components/climate.mqtt/ which does not support templating for the command topics to set the target temperature or mode.
In my case I have an eCozy thermostat named zigbee2mqtt/livingroom/thermostat_right which means I can now publish to the following topic to set the target temperature: zigbee2mqtt/livingroom/thermostat_right/set/occupied_heating_setpoint.

As such I can now control it from homeassitant with the following yaml:

- platform: mqtt
  name: Living room right
  min_temp: 7
  max_temp: 30
  availability_topic: "zigbee2mqtt/bridge/state"
  modes: ["off", "auto", "heat"]
  mode_command_topic: "zigbee2mqtt/livingroom/thermostat_right/set/system_mode"
  mode_state_topic: "zigbee2mqtt/livingroom/thermostat_right"
  mode_state_template: "{{ value_json.system_mode }}"
  current_temperature_topic: "zigbee2mqtt/livingroom/thermostat_right"
  current_temperature_template: "{{ value_json.local_temperature }}"
  temperature_command_topic: "zigbee2mqtt/livingroom/thermostat_right/set/occupied_heating_setpoint"
  temperature_state_topic: "zigbee2mqtt/livingroom/thermostat_right"
  temperature_state_template: "{{ value_json.occupied_heating_setpoint }}"

Next step is to add climate device discovery but that'll be later in a different PR.

[Koenkk]

This is awesome! Could you perhaps update https://github.com/Koenkk/zigbee2mqtt/blob/dev/lib/extension/homeassistant.js with the thermostat auto discover configuration?