Add comment_form_opening_tag and comment_form_closing_tag filters

[powerbuoy]

I'd like to be able to customize the div#respond.comment-respond wrapper around the comment form and replace it with a suitable HTML5 element, but it seems to be one of the few things one _can't_ change within the form.

Therefore I'd like to suggest two filters added to comment-template.php:2451:

apply_filters( 'comment_form_opening_tag', '<div id="respond" class="comment-respond">' );

And comment-template.php:2643:

apply_filters( 'comment_form_closing_tag', '</div>' );

[andraganescu]

Related #41009.