Create a `node-wot` td extensions ontology

While reviewing #735, I was thinking that it might be useful to have a specific node-wot td extensions ontology so that we can experiment correctly with new protocols/frameworks while waiting for them to be correctly integrated inside a protocol binding template document or in the TD ontology. The extension ontology would be mostly for fast prototyping and testing, but it might create confusion for people and it needs to be updated once in a while.

What do you think? Should we always wait for an official integration with the WoT spec or can we adopt an agile dev strategy?