Allow configuration via a constant

[totallyben]

To support our set up we have to make the following code change to allow us to manage custom ips via the constant WP_ROCKET_VARNISH_CUSTOM_IPS.

Would it be possible to include this update in the next release as might be helpful for others?

function set_custom_varnish_ip( $ips ) {

    $ips[] = '127.0.0.1';	// Enter your custom Varnish IP here
    // $ips[] = '13.1.2.3';	// Add each new IP as a new line

    // add any ips specified by WP_ROCKET_VARNISH_CUSTOM_IPS
    if (defined('WP_ROCKET_VARNISH_CUSTOM_IPS')) {
        $ips = array_merge(
            $ips,
            explode(',', str_replace(' ', '', WP_ROCKET_VARNISH_CUSTOM_IPS))
        );
    }
	
    return $ips;
}