custom: Add "exec-on-event" config

This config allows disabling the default behavior of re-executing the
script whenever an event that has a command set is triggered.

Fixes #841

man/waybar-custom.5.scd

@@ -22,6 +22,12 @@ Addressed by *custom/<name>*
 	The path to a script, which determines if the script in *exec* should be executed.
 	*exec* will be executed if the exit code of *exec-if* equals 0.
 
+*exec-on-event*: ++
+	typeof: bool ++
+	default: true ++
+	If an event command is set (e.g. *on-click* or *on-scroll-up*) then re-execute the script after
+	executing the event command.
+
 *return-type*: ++
 	typeof: string ++
 	See *return-type*