Export XHTML Pages

When a Web uses one of the Markdown Text Filters, and you export
all the pages as a zip file, you'd like the MathML and SVG to
render when the pages are viewed locally. This means saving them
with a .xhtml extension. Users of non-XHTML-capable browsers or
Textile users should still get .html files.

app/controllers/application.rb

app/controllers/wiki_controller.rb

lib/chunks/chunk.rb

lib/chunks/literal.rb

lib/url_generator.rb

test/functional/wiki_controller_test.rb