JSONLD: Context Scopes

[jimmarino]

We need to add the ability to define Json-Ld contexts by processing context.

Currently, all namespaces are included when performing compaction, regardless of whether they are referenced in the message body. Having a scoping mechanism will allow namespaces to be registered for specific egress (compaction) points. In addition, we need to add the ability to specify context references that should be included in a scope, not just namespaces.