Include WPThemeReview as an available standard

[carolinan]

WPThemeReview Standard for PHP_CodeSniffer attempts to automate the code analysis part of the WordPress.org Theme Review Process.

https://github.com/WPTT/WPThemeReview

It only has a few sniffs but is helpful for theme developers who submit themes to the official theme directory
(or for default themes required to pass these guidelines).