add "enable continuous messaging"

[Altblechkutscher]

I initially created a feature request https://github.com/nlef/moonraker-telegram-bot/issues/299, but as i surfed the file I found out that the changes are straight forward to do. See the following description:

add "enable continuous messaging" add a feature (which can be set via the configuration file) wheter messages (with photos) should be updated or sent continuous (e.g. always send a new message). If enabled, "last updated" is not being appended to the message.