Added command directive 'title_format'.

This directive will be used to customize the window title.

src/commands.c

@@ -1899,6 +1899,17 @@ void cmd_scratchpad_show(I3_CMD) {
     ysuccess(true);
 }
 
+/*
+ * Implementation of 'title_format <format>'
+ *
+ */
+void cmd_title_format(I3_CMD, char *format) {
+    DLOG("setting title_format to %s\n", format);
+
+    cmd_output->needs_tree_render = true;
+    ysuccess(true);
+}
+
 /*
  * Implementation of 'rename workspace [<name>] to <name>'
  *