Added Filters for button captions and text

Added filters:
'cookie_banner_caption_accept'
'cookie_banner_caption_more_info'
'cookie_banner_text'

This is good for developers who want to translate their WordPress Site. With the Filter Hooks you can easily inject your translations to the buttons,

A Theme developer could write the following in his functions.php

```
function cookie_banner_modify_caption_accept( $caption_accept ) {
    return __('Accept');
}

add_filter( 'cookie_banner_caption_accept', 'cooki_banner_modify_caption_accept' );
```

This will also word for multi-language sites.