v0.2.0

Changes:
  * Breaking Change: Recorder.Add must now return an error. For
    Values.Add, this is always nil.
  * Recorder.Add may now return an error. Reader will automatically
    wrap this as a *RecordingError.
  * SyntaxError now implements Unwrap.
  * Error handling now uses errors.Is, so it's safe for a Reader to
    return wrapped errors.