Add option to output report in machine-readable XML

Right now, it seems that the only machine-readable output option is JSON.

But, since our toolset works on epub, it almost exclusively parses XML. We would like to parse the machine-readable output of Ace, but that means introducing a JSON parser as a dependency--which is a big dependency in our already-heavy toolset. It would also mean that the Ace output parsing branch would have a different pattern and style than the entire rest of our codebase. Future maintainers would have to know the quirks of both XML and JSON, instead of just XML.

Plus, epub is an XML-based format, so it only makes sense that an epub compliance checker would also output XML :)