Option to set a start number

[alifirat]

Hi,

I'm using your tool to make some benchmarks on MQTT.

There is one option missing in your tool setting the start number of a topic.

For example :

./mqtt_bench -start_number=10 to -topic="streaming/foo/ -count=10 -action=p

will publish data in topic : streaming/foo/10, streaming/foo/11, ..., streaming/foo/20.

Can you add a such option ?