This adds a reader and writer for an XML format equivalent to `native`
and `json`.
XML schemas for validation can be found in `tools/pandoc-xml.*`.
The format is documented in `doc/xml.md`.
API changes:
- Add module Text.Pandoc.Readers.XML, exporting `readXML`.
- Add module Text.Pandoc.Writers.XML, exporting `writeXML`.
A new unexported module Text.Pandoc.XMLFormat is also added.
massifrg