Improve input type support.

- Get explicit type from -t/--type options,
- Else: guess type from input location name,
- Else: try JSON,
- Else: try HTML,
- Else: fail.

An edge case is using auto-detection on stdin (or unknown file
extension) with a JSON error. It will fallback to HTML parser and likely
not find RDFa and return empty results instead of a failure.