Add help printout

[sikmir]

• Add help printout

  Usage: wb-mqtt-knx [options]
  Options:
  -d       level     enable debuging output:
                     1 - knx only;
                     2 - mqtt only;
                     3 - both;
                     negative values - silent mode (-1, -2, -3))
  -c       config    config file (default: /etc/wb-mqtt-knx.conf)
  -p       port      MQTT broker port (default: 1883)
  -H       IP        MQTT broker IP (default: localhost)
  -u       user      MQTT user (optional)
  -P       password  MQTT user password (optional)
  -T       prefix    MQTT topic prefix (optional)
  -k       url       KNX url (default: ip:localhost:6720)

• Add -u/-P/-T mqtt arguments
• Add -c argument to specify custom config file
• Fix debug logging level setting (mqtt logging)