Implement debuglog command

Add debuglog command that takes toggle|on|off. Add get_debug_logging()
to be able to toggle. Make t/187-commands-parser.t expect 'debuglog'.
Document the debuglog command in userguide.

docs/userguide

@@ -1762,6 +1762,8 @@ stack-limit rows 5
 image:stacklimit.png[Container limited to two columns]
 ///////////////////////////////////////////////////////////////////////////////
 
+[[shmlog]]
+
 === Enabling shared memory logging
 
 As described in http://i3wm.org/docs/debugging.html, i3 can log to a shared
@@ -1787,6 +1789,24 @@ bindsym $mod+x shmlog toggle
 i3-msg shmlog $((50*1024*1024))
 ---------------
 
+=== Enabling debug logging
+
+The +debuglog+ command allows you to enable or disable debug logging at
+runtime. Debug logging is much more verbose than non-debug logging. This
+command does not activate shared memory logging (shmlog), and as such is most
+likely useful in combination with the above-described <<shmlog>> command.
+
+*Syntax*:
+------------------------
+debuglog <on|off|toggle>
+------------------------
+
+*Examples*:
+------------
+# Enable/disable logging
+bindsym $mod+x debuglog toggle
+------------
+
 === Reloading/Restarting/Exiting
 
 You can make i3 reload its configuration file with +reload+. You can also