Added command directive 'title_format'.

This directive will be used to customize the window title.

include/commands.h

@@ -276,6 +276,12 @@ void cmd_move_scratchpad(I3_CMD);
  */
 void cmd_scratchpad_show(I3_CMD);
 
+/**
+ * Implementation of 'title_format <format>'
+ *
+ */
+void cmd_title_format(I3_CMD, char *format);
+
 /**
  * Implementation of 'rename workspace <name> to <name>'
  *