Mqtt items and discovery

[DaveNeudoerffer]

Here it is -- mqtt items and discovery.

I have just submitted a pull request that adds mqtt item support as well as mqtt discovery as defined by
Home Assistant.  Full documentation is found in mqtt_items.pm, but I summarize here.

mqtt_items.pm:

• An mqtt item is a misterhouse object representing the mqtt device,
    managing all messages associated with that device.  
• There are mqtt_RemoteItems for mqtt devices.  The item will receive state messages
    from the device and publish command topics to the device.  This  
    supports Tasmota, IOT4, and likely ESPurna devices
• Also supported are mqtt_LocalItems which are linked to an existing
    misterhouse item and will publish state messages and receive command messages
    for that existing item
• MQTT LWT messages are also supported for online/offline status as
    well as other topics such as the brightness topic for dimmable devices.  I have not yet added support for colour lights, but if someone can  point me at a good MH implementation of a light with colour, I will take a look.
• MQTT types supported: switch, light, binary_sensor, sensor
• .mht support for MQTT_REMOTEITEM, MQTT_LOCALITEM and MQTT_INSTMQTT
• MQTT_REMOTEITEMS support Tasmota, IOT4, and likely ESPurna devices
• MQTT_LOCALITEM supports almost any existing mh device that can be mapped to an MQTT type
• MQTT_INSTMQTT supports devices published by the insteon-mqtt project
• These are statically defined MQTT objects, and do not need discovery, but
    defining the objects statically can be tedious.  See discovery below.

mqtt_discovery.pm:

• Discovery allows for the processing of discovery messages as published by
    mqtt devices.  Most mqtt devices have some support for discovery as
    defined by Home Assistant.  Usually it needs to be turned on on
    the device (eg. Tasmota -- setOption19 1).  
    Misterhouse items are created (mqtt_RemoteItem) for
    the discovered devices.  These items can be written to a .mht file
    so the next time misterhouse is run, they will appear as first class
    misterhouse items.    
• This module will also publish discovery information for mqtt_LocalItems,
    allowing misterhouse items to be discovered by Home Assistant, or by another
    instance of misterhouse or anything else that can consume discovery messages.
• I have tried this with TASMOTA and IOT4 devices, but it should work
    much more broadly.
• .mht support for MQTT_DISCOVERY class to process discovery messages and
    MQTT_DISCOVERED_ITEM item type for discovered items written to a .mht
    file by mqtt_Discovery::write_discovered_items().
   
  mqtt.pm:
• These new modules use the existing broker support in mqtt.pm.  
• I have tried very hard not to break anything in the existing mqtt.pm,
    including the existing mqtt_item class, but there have been some
    bug fixes and small enhancements made to support the new functionality:
      - multiple mqtt broker support has been fixed.  The code was there,
        but it shared a receive buffer across brokers which didn't work very well :-)
      - enhanced debugging support with debug levels 1-3
      - the ability for an object to listen to a list of topics or topic patterns.
        (thanx Gilles for the topic pattern support!)
      - I had to remove Gilles' short circuit when an object was found matching
        a message topic because there are cases where multiple objects receive the
        same topic
      - optionally unloaded the set function, with the addition of a new object
        method $obj->receive_mqtt_message().  This makes the item code more understandable.
      - added methods for managing retained topics
       
  Enjoy.  Open to any and all feedback or suggestions.

[hplato]

Thanks Dave!