v1.0.0-rc.2

Switched from beta to rc because this really is nearly ready to publish.

• Documentation is now at https://eemeli.org/yaml/, rather than partly in README files and partly missing.
• Rethink semantic/syntax error boundary: Instead of classifying based on source API level, syntax errors now result in broken data while semantic errors "only" affect metadata.
• Add public re-exports: map, pair, parse-ast, scalar, seq, types/binary, types/timestamp
• Specify node ≥ 6 dependency (PR #11)
• Please ignore rc.1, it was missing a minor package.json update