Simple command-line MQTT client

Command-line args follow those for mosquitto_sub.

Commands:

1. qos QOS: Set the QoS for subsequent operations.
2. sub TOPIC: Subscribe to the given topic.
3. unsub TOPIC: Unsubscribe from the given topic.
4. retain (true|false): Set retain flag for subsequent pubs.
5. pub TOPIC MSG: Publish MSG to the given TOPIC.
6. sleep DURATION: Sleep for the given duration (e.g, '1s').

By default (controlled by -sh), input lines are shell-expanded: Each substring of the form <<SHELL_COMMAND>> is replaced by the stdout of executing SHELL_COMMAND.