Just merged https://github.com/nostr-protocol/nips/pull/1185, which introduces NIP 73 for external identities via the `i` tag. A different form of them is already in use in NIP 39, for linking external identities to user profiles.

NIP 73 IDs are instead fully-qualified, and include a second parameter which is a suggested URL, which allows clients to easily link out when they don't want to special case every ID type. 

What's neat about this is that this will allow platforms like Fountain to reference podcast episodes in nostr comments, which can be displayed and linked out to by regular nostr clients, expanding the scope of the nostr-enabled web!