Fixed a bug related to loading screen/added custom script function

【Contents of the fix】
■Fixed a bug that the smooth scrolling of links in the page does not work when using “Show loading screen” in the basic settings of theme options.
■Added “Custom Scripting” to the basic theme option settings.

【Modified file】
admin/theme-options.php
functions/head.php
languages/ja.mo
languages/ja.po
style.css