These filters will allow WordPress plugins to modify the cookies and
plugins lists used by WP Super Cache instead of the add/delete functions
introducted in #574 and #580.
Duplicate entries are removed so the filter function does not need to
worry about that. Example code:
`function myplugin_add_wpsc_cookies_filter( $cookies ) {
    $cookies[] = 'myplugin';
    return $cookies;
}
add_filter( 'wpsc_cookies', 'myplugin_add_wpsc_cookies_filter' );`
`function myplugin_delete_wpsc_cookies_filter( $cookies ) {
    if ( in_array( 'myplugin', $cookies ) ) {
        unset( $cookies[ array_search( 'myplugin', $cookies ) ] );
    }
    return $cookies;
}
add_filter( 'wpsc_cookies', 'myplugin_delete_wpsc_cookies_filter' );`

…tings (#582)
This PR adds four actions to modify wpsc_cookies and wpsc_plugins:
* wpsc_add_plugin
* wpsc_delete_plugin
* wpsc_add_cookie
* wpsc_delete_cookie
These actions will allow WordPress plugins to modify the cookies and
plugins lists used by WP Super Cache instead of the add/delete functions
introduced in #574 and #580.
Duplicate entries are removed. Example code:
For example, to add a cookie name called 'euCookie':
`do_action( 'wpsc_add_cookie', 'euCookie' );`
To remove that cookie:
`do_action( 'wpsc_delete_cookie', 'euCookie' );`