Add options for MQTT username/password

[geoffbon]

This looks like a great solution, and it's really close to working for me, except that my MQTT server is set up with user/password authentication.

It looks like the paho library supports this via MQTTClient_createWithOptions(), but I'm not quite c-savvy enough to create the right pull request. Any chance you can add this?
Perhaps adding a couple of optional parameters on the server address line to prevent breaking existing instances, i.e. allow both these to work:

tcp://172.148.5.11:1883
tcp://172.148.5.11:1883,jack,secretpassword

Otherwise I'll need to reconfigure stuff to allow non-authenticated connections, which I'd rather not do.

[tconnz]

+1 for username/password if at all possible and "spare time" allows.

Many thanks for this program too @deepcoder