Simplify extending/chaining JSON-LD contexts

[apirogov]

Currently, when extending a semantic schema, the @context must be completely overwritten to modify it.
Might be useful to have a "smart" helper that can extend an existing context.

Apparently one can simply use a list of context objects. What is the semantics of that in JSON-LD? Is there an order? What happens if definitions conflict?

If the default semantics is compatible with "overriding", we could use that. Otherwise it would be quite some work to support so we rather not do it (we don't resolve external context URLs so these are opaque).

Maybe its not such a bad idea to force people downloading the JSON-LD context and maybe even introducing a schema for "contexts" (basically a dict wrapper).

This needs much more thought.