Make the metadata extractor accept rdfa-website test runner style arguments

[rhdunn]

At the moment, the metadata application cannot be used directly as a runner for the rdfa-website test suite (via the bin/run-suite command). This is due to two reasons:

1. The reader API does not accept URIs (see issue Support opening documents from a URI #51).
2. The metadata command does not accept the command-line options used by the bin/run-suite script.

To work around this, the tests/rdfa-test-harness script has been created. This should not be needed as the metadata command-line tool should recognise these arguments (or ignore them if they are not relevant).

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