added wp_post_revision_meta_value filter

[petenelson]

For issue #24, sample of using the new filter

add_filter( 'wp_post_revision_meta_keys', function( $keys ) {
    $keys[] = 'preview_twitter_description';
    return $keys;
} );

add_filter( 'wp_post_revision_meta_value', function( $meta_value, $meta_key, $new_autosave, $posted_data ) {

    // Save the preview Twitter description as a separate piece of post meta
    if ( 'preview_twitter_description' === $meta_key && isset( $posted_data['yoast_wpseo_twitter-description'] ) ) {
        $meta_value = 'Twitter: ' . sanitize_text_field( $posted_data['yoast_wpseo_twitter-description'] );
    }

    return $meta_value;
}, 10, 4 );

[grappler]

@adamsilverstein Did you close this PR by mistake?

I think this is a needed addition. I have named the filter wp_post_revision_autosave_meta_value in my fork of the plugin.

[adamsilverstein]

It was closed because I renamed the master branch to main, apologies. Reopening and will work on this soon!

[adamsilverstein]

@grappler when I tried to re-open it says "the repository that submitted this pull request has been deleted. " I can probably reconstruct from the diff here, but feel free to reopen or open a replacement PR.

I'll also try to review any PRs that were closed by my changing the base branch and reopen those.

[grappler]

I can open a replacement PR once #61 is merged as the changes are need in the same lines.