add parameters support to getContentFunc Minify_Source callbacks

This is useful for getContent in Minify_Source if you need to pass extra data when building response content. Without this, I would have to duplicate logic in `foo`, so this would help to reuse existing variables, objects... 

```php
    public static function foo($configpath, $app, $opts = array()) {
        /* @var $config */
        require $configpath;
        $cb = foobar::create($config, $app);
        $lastModified = Minify_HTML_Helper::getLastModified($cb->getDependencies());

        $config['load_module'] = !empty($_GET['cb']) ? (string)$_GET['cb'] : $config['instance'];

        $opts = array(
            'id' => $configpath . ':' . $app . ':'. $config['load_module'],
            'lastModified' => $lastModified,
            'getContentFunc' => array(__CLASS__, 'foo_callback'),
            'getContentFuncParams' => array(
                $cb,
                $config,
            ),
        );

        $file = new Minify_Source($opts);
        return $file;
    }

    public static function foo_callback($id, $cb, $config) {
        return $cb->get($config);
    }
```