feature request: Add custom classes to the buttons

[nicdim]

A feature request:
To have the ability to enter additional class names in wp-admin, to be added to the class attribute for the buttons in the cookie notice banner.
These can be added in addition to the current css styles: none/wordpress/bootstrap

I've already made the modifications necessary but can't seem to create a pull request to submit my changes. Would you like my contribution? (changes to file: cookie-notice.php)

[bartoszarendt]

Done in recent release.