Add possibility to configure publish options in MQTT output plugin

[tbarthelet]

Hi,
Similarly to issue #172 it would be very useful to be able to set publish options when publishing a message to mqtt.
For example, it allows to define QoS or if message should be retained.
See https://www.npmjs.com/package/mqtt#publish for all details.

This could be done by using this.client.publish(topic, msg, this.config.publishOptions) instead of this.client.publish(topic, msg) in mqtt.js and setting options in the mqtt output module accordingly.

[otisg]

Any chance you can submit the PR, @tbarthelet ?

[tbarthelet]

OK, I'll double check for a working example and submit a PR.

[adnanrahic]

Thanks a lot @tbarthelet ! Feel free to add me as a reviewer. I'll check it out ASAP.