Add feature to throw error when unmapped elements are dropped during processing

[dlongley]

When certain elements in a document cannot be mapped to absolute URLs (via @context or a base URL), then are silently dropped during expansion per the spec. There should be a feature in this processor that, instead, causes an error to be raised. This is particularly important when this library is used in conjunction with digital signing libraries.

Related to #178.

[dlongley]

Partially supported with expansionMap and compactionMap here: bcae961

Still need something for toRDF.

[dlongley]

Specifically, when converting to RDF, relative URIs are dropped and this should be handled by another map or similar mechanism.