Support for custom MFN posts and featured images

• Extends upsertAttachment to optionally upload image attachments to the wordpress media library
• Enabled thumbnail support for our custom post type
• Option to delete all attachments with posts
• Add 'mfn_attachment_file_title' filter to override file_title
• posthook: Support 'X-Hub-Ext-Ping' Websub 'extension'
• Tweaks/fixes for thumbnail support, sync and ping feature
• Change default URLs to feed API
• Allow adding own MFN posts via WP-admin 'MFN News Items'
• Added check for IR-PR type tags to prevent both being checked at the same time