Improved MQTT status messaging and webRequest URI bug fix

Breaking changes

• MQTT online and offline status messages are now sent when the service starts or fails
  to start. This, combined with the LWT message, makes it easy to set up MQTT binary sensors in Home Assistant
  to track the status of the system and send notifications to people if the system goes down
  or isn't running. This is a breaking change if you rely on the LWT message.
  The format of the offline message sent for the LWT changed to align with the online and processing status messages. See the wiki for documentation on the status message format. Resolves issue182.
• webRequest URIs are no longer double-encoded. Instead only the text replaced with a
  mustache template is encoded. This is a breaking change if you had previously modified your webRequest URIs to work
  around issue 176. If you previously worked around the bug by removing encoding from the URIs in the trigger configuration
  file you will need to put the encoding in again. Resolves issue 176.

Other changes

• MQTT detection messages now include a friendly formatted version of the predictions,
  for example: "formattedPredictions": "dog (98%)". Resolves issue 181.
• Failed calls to the Deepstack server no longer throw an unhandled promise rejection
  exception. Resolves issue 175.