Add a writer for PreTeXt

[oscarlevin]

It would be great to convert to PreTeXt. It would go in as another XML format, but unlike DocBook, PreTeXt has excellent math (latex) support, among other nice features. Info about the project here.

A custom lua writer is already available, but this has sloppy indenting and sectioning, which I think would be fixed with a normal writer.

See discussion on the google group.

[jgm]

Thanks for developing the custom writer; that should make it easy to add a real writer.