i3-msg: add support for SUBSCRIBE message type

If i3-msg is invoked with -t subscribe, it will wait for the first event
matching the given payload, before exiting.

For instance, get the number of the next focused workspace with:

    i3-msg -t subscribe '[ "workspace" ]' | jshon -e current -e num

Like inotifywait, the -m flag allows to wait indefinitely for events,
instead of exiting right after receiving the first one.

For example, continuously monitor the names of focused windows with:

    i3-msg -t subscribe -m '[ "window" ]' | jq .container.name