Adds retain support for publishing messages

[Fapiko]

This adds retains support to published messages. It should be backwards compatible with existing clients as it defaults to false if not supplied.

Solves #20 and #161

[Paulie92]

Is there any example how to call this function in the code? unfortunately I have no idea how can I use it.

Thank you

[trickkiste]

When will this be merged? Seems all tests have passed and it is backward compatible, so what is the holdup? The feature is sorely needed!

Please merge!

[ben-willmore]

I made an up-to-date version of this which overloads the publish and publishPacket functions rather than using optional arguments:

https://github.com/ben-willmore/Adafruit_MQTT_Library/tree/retain-flag

[brentru]

@ben-willmore Are you interested in checking out a pull request against this library, I'll review it.

[ben-willmore]

@brentru Sure, have now done so -- #216

[brentru]

Resolved by https://github.com/adafruit/Adafruit_MQTT_Library/pull/216