support overriding config file in shell mode

[mammothbane]

Problem or use case

I want to connect to different brokers via mqtt shell in different situations (e.g. different projects, local dev vs. staging environments, etc.) and it'd be nice not to have to type out all of my connection parameters in connect every time.

The current config file in ~/.mqtt-cli only allows me to specify parameters for a single broker at a time, and is inherently global to my entire user.

Preferred solution or suggestions

I'd like to be able to pass a config file as a command line argument, e.g.

$ mqtt shell [-c|--config] path/to/config.properties