Add option to not load external CSS

Currently, by adding the attribute data-css-override it is possible to add a custom stylesheet which, to my understanding, does not replace the main one. If I want to remove all the default Commento CSS altogether, so that the client does not have to make any other request to the Commento server, it should be possible to set data-css-override=false or data-css-override=nil to tell Commento not to load any CSS remotely, and use the page CSS instead.