Add ability to provide a global engine extension through Express setting...

Hi,

Since Express is supposed to support custom extension through the setting 'view engine', this patch only adds the feature to adaro (http://expressjs.com/3x/api.html#app-settings).
(It might be handled during the init as well, with a config ext parameter)

This also does not break the existing behavior. However I don't really see any advantage using 'ext' as a render option. I mean, if you need to precise the extension each time you call render, you could just set the extension in the template name i.e. `render('mytemplate.dust')` rather than `render('mytemplate', { ext: 'dust' })`.