[Suggestion] Add filters for notification title, body and url

It would be useful to be able to filter the title, body and url of a notification. This could be added to lines 58 – 60 of includes/metabox.php.

The reason for this is to be able to customise the output based on the post. For instance, a certain post type may not be public, but may be displayed as part of a custom set of results on a page. Having a filter available for the url would allow for the notification to link to the page rather than the (unavailable) post permalink in this case.

Being able to filter the title and body on a post-by-post basis would also be useful.

Thanks!