Supports setting the retain flag on published messages

Hi, 

I sometimes find it useful to publish messages to a MQTT broker with the retain flag set. This saves clients having to wait for a new message to be published, they always get sent the last retained message upon connect, if the retain flag is True. This makes this PR makes that a user configurable option in weather.ini. If nothing is set it defaults to not publishing retained messages and doesn't require the user to do anything.