Changes since v0.2.0:
- Add class for RdfaBlock
- Consistent enrichment of rich nodes with RDFa attributes by the context scanner
- Move RDFa related helpers to a separate rdfa-helpers file

Deprecations:
- rdfaBlock.richNode: use rdfaBlock.richNodes instead
- resolvePrefixes from rdfa-context-scanner: use resolvePrefixedAttributes from rdfa-helpers instead