Record comment meta in wp_new_comment

[tellyworth]

Spam filter plugins usually trigger on the preprocess_comment filter, which obviously happens prior to storing the comment (and thus prior to knowing comment_ID).

Comment meta would be ideal for storing spam status information, but since the comment_ID isn't known at that point, doing so involves jumping through some hoops.

This patch provides a simple way for spam filters to include comment metadata from preprocess_comment. They simply include values in a 'comment_meta' array like this:

add_action('preprocess_comment', 'my_spam_handler');

function my_spam_handler($comment) {
    $comment['comment_meta']['my_spam_status'] = 'not-spam';
    return $comment;
}

Each key/value pair in the comment_meta array is stored as comment metadata.