- feature: add variables to customize the data fiels in the
  print hooks, default values are backwards compatible
- feature: add messages on connecting/disconnecting to mqtt
- feature: add script unload hook function to clean up the mqtt
  connection
- feature: add variable for mqtt client name
- feature: add buffer_long and buffer_full in message json
- fix: move mqtt connect code to global scope, avoiding
  new connections on each message causing lost messages due to
  network/socket errors, and optimized for mqtt client
  loop_start/stop
- fix: rename mqtt_timeout to mqtt_keepalive