Using different hook for better compatibility

Hey!

I propose an improvement to this great plugin. For better compatibility with all the themes, I recommend hooking to comment_form_fields filter instead of comment_form_default_fields which is used now.

From https://developer.www.ads-software.com/reference/functions/comment_form/ it is apparent that when the theme sets is own 'fields' in the $args the checkbox is not shown.

I can provide a PR if needed.

Let me know! Thank you.