Take advantage of PHPCS and WPCS goodies

#659 almost accidentally added an instance of short array syntax, but PHPCS could have caught it for us: https://github.com/squizlabs/PHP_CodeSniffer/issues/483#issuecomment-83225657. We might want to enable that sniff as an `error`.

There are also some WordPress-specific PHPCS properties we could set: [text domain](https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/wiki/Customizable-sniff-properties#internationalization-setting-your-text-domain),
[naming prefixes](https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/wiki/Customizable-sniff-properties#naming-conventions-prefix-everything-in-the-global-namespace), and [minimum WordPress version](https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/wiki/Customizable-sniff-properties#minimum-wp-version-to-check-for-usage-of-deprecated-functions-classes-and-function-parameters).