feat(script): Configurable condition using exec-if

Add new config parameter `exec-if = cmd` that needs to
exit successfully before the main exec command will be invoked.

Closes #298

include/modules/cmdscript.hpp

@@ -11,6 +11,7 @@ namespace modules {
 
    protected:
     void process();
+    chrono::duration<double> sleep_duration();
   };
 }