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

man/i3-msg.man

@@ -31,6 +31,11 @@ with an error.
 *-t* 'type'::
 Send ipc message, see below. This option defaults to "command".
 
+*-m*, *--monitor*::
+Instead of exiting right after receiving the first subscribed event,
+wait indefinitely for all of them. Can only be used with "-t subscribe".
+See the "subscribe" IPC message type below for details.
+
 *message*::
 Send ipc message, see below.
 
@@ -75,6 +80,11 @@ Gets the currently loaded i3 configuration.
 send_tick::
 Sends a tick to all IPC connections which subscribe to tick events.
 
+subscribe::
+The payload of the message describes the events to subscribe to.
+Upon reception, each event will be dumped as a JSON-encoded object.
+See the -m option for continuous monitoring.
+
 == DESCRIPTION
 
 i3-msg is a sample implementation for a client using the unix socket IPC
@@ -91,6 +101,9 @@ i3-msg border normal
 
 # Dump the layout tree
 i3-msg -t get_tree
+
+# Monitor window changes
+i3-msg -t subscribe -m '[ "window" ]'
 ------------------------------------------------
 
 == ENVIRONMENT