Include triples which changed in an RDF resource in the notification

[brownhoward]

Suggest a level 2 protocol in which the triples which changed in an RDF resource are included in the notification.

Originally posted by @timbl in solid/notifications-panel#3 (comment)

[elf-pavlik]

Today we discussed that security considerations may differ for a subscription that only provides IRIs from those which include (partial) representation of resources. This might be relevant especially for subscriptions type relying only on Capability URLs for securing notifications sources.

[elf-pavlik]

first draft in PR #61