Enhancement: More flexible bootstrap

This will allow to initialize plugin this way: 

```php
new CssInlinerPlugin([
        'cleanup' => true,
	'useInlineStylesBlock' => true,
	'stripOriginalStyleTags' => true,
]);
```

Actually I suggest it for compatibility with Yii2 framework. With this change it's possible to enable CssInlinerPlugin in this easy way in Yii2 config file: 

```php
return [
    'components' => [
    
        [...]
        
        /**
         * Mailer
         *
         * @link https://github.com/yiisoft/yii2-swiftmailer
         * @link http://swiftmailer.org/docs/messages.html
         */
        'mail' => [
            'class' => 'yii\swiftmailer\Mailer',
            'messageConfig' => [
                'charset' => 'utf-8',
            ],
            'transport' => [
                'class' => 'Swift_SmtpTransport',
                'host' => 'mail.domain.com',
                'username' => 'support@domain.com',
                'password' => '...',
                'plugins' => [
                    [
                        // connect CssInlinerPlugin
                        'class' => 'Openbuildings\Swiftmailer\CssInlinerPlugin',
                        // pass arguments to CssInlinerPlugin's constructor
                        'constructArgs' => [[
                            'cleanup' => true,
                            'useInlineStylesBlock' => true,
                            'stripOriginalStyleTags' => true,
                        ]],
                    ],
                ],
            ],
        ],
    ],
];

```

Existing implementation when CssToInlineStyles object is passed to class constructor as parameter is already perfect.

I believe this change will be good addition that will allow to use plugin in more flexible way.