Add support for instagram embed URLs (iframe)

We ran into an issue where the media helper detects instagram "embed" URLs as regular Instagram images which overrides passing the type as "iframe". This patch adds:
- support for embed URLs (instagram.com/p/<ID>/embed/) and also provides some sensible defaults for sizing Fancybox to fit the contents
- An extra "opts" object can be added to media helper configs to allow arbitrary options to be merged on to the current item (width and height in this case)