Updated xml.cson to support XLIFF

XLIFF is an XML-based format created to standardize the way localizable data are passed between tools during a localization process, according to [Wikipedia](http://en.wikipedia.org/wiki/XLIFF). The main extension is `.xlf`, but some tools are using `.xliff` (the Symfony2 translator for example). So these two extensions should be supported as part of the standard XML grammar.