Add preview URL filter

[alecgeatches]

Description

See discussion in #70. It can be difficult to set up home_url() to point to a decoupled front end for previews, but ensure WordPress continues to function normally. For flexibility, we're adding this filter to modify the preview URL destination separately:

/**
 * Filter preview URL before redirect.
 *
 * @param  $preview_url     Generated preview URL, based on home_url().
 * @param  $preview_token   Token used to view the preview.
 * @param  $post_id         ID of the post being previewed.
 */
$preview_url = apply_filters( 'vip_decoupled_preview_url', $preview_url, $preview_token, $post->ID );

Note that wp_safe_redirect() has also been changed to wp_redirect(), as the former will not allow redirection outside of the home domain.