Add SCRIPT_DEBUG constant to allow loading of optimised CSS/JS.

Adds the constant if it has been defined in the env file, otherwise defaults to true on development and false on staging and prod.

Theme developers using a build script for their CSS/JS can use this to enqueue the un-minified version of scripts for debugging during development and switch to the concatenated/minified version on staging and production. For example:

```php
function my_enqueue_function() {

	if ( defined('SCRIPT_DEBUG') && SCRIPT_DEBUG ) {
		wp_enqueue_style( 'example-lib-1' );
		wp_enqueue_style( 'style-dev' );

		wp_enqueue_script( 'jquery' );
		wp_enqueue_script( 'app' );
	} else {
		wp_enqueue_style( 'build-css' );
		wp_enqueue_style( 'build-js' );
	}
}
```

More info on [SCRIPT_DEBUG](http://codex.wordpress.org/Editing_wp-config.php#Debug)